Firefox CSS Modification (up to 140 ESR)

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The following modifications are based on Aris-t2’s CustomCSSforFx project.

For Firefox versions beyond 140, please see this page for reference.

A Firefox 140 with CSS modifications.

Modifications of CustomCSSforFx’s Original CSS Files

appbutton/appbutton_in_titlebar.css (CSS ver: 4.7.3; Firefox ver: 113–140)
  --appbutton_in_titlebar: 92px !important;

  height: 2rem !important;

  padding-inline-start: 0.81rem !important;
  padding-inline-end: 2.19rem !important;
  height: 2rem !important;

  margin-top: 5px !important;
generalui/popup_compact_menus.css (CSS ver: 4.7.3; Firefox ver: 102–140)
  --item_y_padding: 2px; /* old value: 2px */

  menupopup > menuitem,
  menupopup > menu {
	padding-block: 0 !important;
	padding: 0.08rem 0 0.29rem !important;
	padding-inline-start: 4px !important;
	padding-inline-end: 4px !important;
  }

  :is(menupopup, panel) :is(.menu-accel,.menu-iconic-accel) {
	margin-inline-end: 4px !important;
  }
tabs/tab_audio_icon.css (CSS ver: 4.7.3; Firefox ver: 136–140)
.tab-audio-button {
  --button-size-icon-small: 22px !important;
  --button-min-height-small: 22px !important;
  --button-border-color-ghost: unset !important;
  --button-border-radius: 0 !important;
  background: rgba(255,255,255,0) !important;
  border-radius: 0 !important;
  scale: .9 !important;
  transform: translateY(-1px);
  margin-inline-start: -2px !important;
}
tabs/tab_context_menuitems_visibility.css (CSS ver: 4.7.3; Firefox ver: 102–140)
/* hide 'Reload Tab' menuitem */
#tabContextMenu #context_reloadTab {
  visibility: collapse !important;
  display: none !important;
}

/* hide 'Mute Tab' menuitem */
#tabContextMenu #context_toggleMuteTab {
  visibility: collapse !important;
  display: none !important;
}

/* hide separator after 'Mute Tab' menuitem */
#tabContextMenu #context_toggleMuteTab + menuseparator {
  visibility: collapse !important;
  display: none !important;
}

/* hide 'Duplicate Tab' menuitem */
#tabContextMenu #context_duplicateTab {
  visibility: collapse !important;
  display: none !important;
}

/* hide 'Move to New Window' menuitem */
#tabContextMenu #context_openTabInWindow {
  visibility: collapse !important;
  display: none !important;
}

/* hide separator after 'Move to New Window' menuitem */
#tabContextMenu #context_sendTabToDevice_separator {
  visibility: collapse !important;
  display: none !important;
}

/* hide 'Send Tab to Device' menuitem */
#tabContextMenu #context_sendTabToDevice {
  visibility: collapse !important;
  display: none !important;
}

/* hide separator after 'Send Tab to Device' menuitem */
#tabContextMenu #context_sendTabToDevice + menuseparator {
  visibility: collapse !important;
  display: none !important;
}

/* hide 'Reload All Tabs' menuitem */
#tabContextMenu #context_reloadAllTabs {
  visibility: collapse !important;
  display: none !important;
}

/* hide 'Bookmarks All Tabs...' menuitem */
#tabContextMenu #context_bookmarkAllTabs {
  visibility: collapse !important;
  display: none !important;
}

/* hide 'Close Tabs to the Right' menuitem */
#tabContextMenu #context_closeTabsToTheEnd {
  visibility: collapse !important;
  display: none !important;
}

/* hide 'Close Other Tabs' menuitem */
#tabContextMenu #context_closeOtherTabs {
  visibility: collapse !important;
  display: none !important;
}

/* hide separator after 'Close Other Tabs' menuitem */
#tabContextMenu #context_closeOtherTabs + menuseparator {
  visibility: collapse !important;
  display: none !important;
}

/* hide 'Reload Selected Tab' menuitem */
#context_reloadSelectedTabs {
  visibility: collapse !important;
  display: none !important;
}

/* hide 'Close Tab' menuitem */
#tabContextMenu #context_closeTab {
  visibility: collapse !important;
  display: none !important;
}

/* hide 'Bookmark Tab' menuitem */
#tabContextMenu #context_bookmarkTab {
  visibility: collapse !important;
  display: none !important;
}

/* hide 'Select All Tabs' menuitem */
#tabContextMenu #context_selectAllTabs {
  visibility: collapse !important;
  display: none !important;
}

/* hide 'Move Tab' menuitem */
#tabContextMenu #context_moveTabOptions {
  visibility: collapse !important;
  display: none !important;
}
toolbars/toolbar_context_menuitems_visibility.css (CSS ver: 4.7.3; Firefox ver: 102–140)
/* hide 'Pin to Overflow Menu' menuitem */
#toolbar-context-menu .customize-context-moveToPanel {
  visibility: collapse !important;
  display: none !important;
}

Additional File (Re)Placements

For Firefox 133 and above (currently tested up to 140), place the following CSS file in /chrome/css/ and then include it in my_userChrome.css with @import "./css/additional_menubar_tabstoolbar_adjustments_133.css";.

additional_menubar_tabstoolbar_adjustments_133.css (CSS ver: 4.7.3; Firefox ver: 133–140)
#main-window #toolbar-menubar:is([autohide="false"], :not([inactive="true"])) ~ #TabsToolbar,
#main-window:not([tabsintitlebar], [customtitlebar]) #TabsToolbar {
	padding-inline-start: 0 !important;
}
#main-window:is([tabsintitlebar], [customtitlebar]):is([sizemode="maximized"], :not([sizemode="maximized"])):is([uidensity=compact], :not([uidensity=compact])) #toolbar-menubar[autohide="true"][inactive="true"] ~ #TabsToolbar {
	padding-inline-start: calc(var(--appbutton_in_titlebar) - 0.5rem) !important;
}
#main-window:is([tabsintitlebar], [customtitlebar]):is([sizemode="fullscreen"], [sizemode="maximized"]) #toolbar-menubar:is([autohide="true"]:not([inactive="true"]), [autohide="false"]) {
	padding-top: 0.25rem !important;
}
#main-window:is([tabsintitlebar], [customtitlebar]) #toolbar-menubar:is([autohide="true"]:not([inactive="true"]), [autohide="false"]) ~ #TabsToolbar {
	margin-left: -0.064rem;
}
#main-window:is([tabsintitlebar], [customtitlebar]):is([sizemode="fullscreen"], [sizemode="maximized"]) #toolbar-menubar:is([autohide="true"][inactive="true"], [autohide="false"]) ~ #TabsToolbar,
#main-window:is([tabsintitlebar], [customtitlebar]):not([sizemode="fullscreen"], [sizemode="maximized"]) #toolbar-menubar[autohide="false"] ~ #TabsToolbar {
	margin-top: -0.15rem !important;
}
#main-window:is([tabsintitlebar], [customtitlebar]):is([sizemode="fullscreen"], [sizemode="maximized"]) #toolbar-menubar[autohide="true"]:not([inactive="true"]) ~ #TabsToolbar {
	margin-top: -0.9rem !important;
}
#main-window:not([tabsintitlebar], [customtitlebar])[sizemode="fullscreen"] #toolbar-menubar:not([inactive="true"]) ~ #TabsToolbar {
	margin-top: -0.25rem !important;
}
#main-window:is([tabsintitlebar], [customtitlebar]):not([sizemode="fullscreen"], [sizemode="maximized"]) #toolbar-menubar[autohide="true"][inactive="true"] ~ #TabsToolbar {
	margin-top: -0.2rem !important;
}
#main-window:is([tabsintitlebar], [customtitlebar]):not([sizemode="fullscreen"], [sizemode="maximized"]) #toolbar-menubar:is([autohide="true"]:not([inactive="true"]), [autohide="false"]) {
	padding-top: 0.2rem !important;
}
#main-window:is([tabsintitlebar], [customtitlebar]):not([sizemode="fullscreen"], [sizemode="maximized"]) #toolbar-menubar[autohide="true"]:not([inactive="true"]) ~ #TabsToolbar {
	margin-top: -0.95rem !important;
}
#main-window:not([tabsintitlebar], [customtitlebar])[sizemode="fullscreen"] #toolbar-menubar[inactive="true"] ~ #TabsToolbar {
	margin-top: -0.3rem !important;
}
#main-window:not([tabsintitlebar], [customtitlebar]):not([sizemode="fullscreen"]) #toolbar-menubar {
	padding: 0.15rem 0 0.2rem 0.2rem !important;
}
#main-window:not([tabsintitlebar], [customtitlebar])[sizemode="fullscreen"] #toolbar-menubar {
	padding: 0.15rem 0 0 0.2rem !important;
}
#main-window:not([tabsintitlebar], [customtitlebar])[sizemode="fullscreen"] #toolbar-menubar[autohide="true"]:not([inactive="true"]) ~ #TabsToolbar {
	margin-top: -0.4rem !important;
}
#main-window:not([tabsintitlebar], [customtitlebar]) #toolbar-menubar[inactive="true"] ~ #TabsToolbar {
	margin-top: -0.35rem;
}
#main-window:not([tabsintitlebar], [customtitlebar])[sizemode="fullscreen"] #toolbar-menubar[inactive="true"] ~ #TabsToolbar {
	margin-top: -0.3rem;
}
#main-window:not([tabsintitlebar], [customtitlebar]):is([sizemode="fullscreen"], [sizemode="maximized"]) #TabsToolbar {
	margin-left: -0.1rem;
}
#main-window:not([tabsintitlebar], [customtitlebar]):not([sizemode="fullscreen"], [sizemode="maximized"]) #TabsToolbar {
	margin-left: -0.08rem;
}
#main-window[sizemode="fullscreen"] #toolbar-menubar[autohide="false"] ~ #TabsToolbar .titlebar-buttonbox-container {
	visibility: collapse !important;
	display: none !important;
}

For Firefox 118 to 132, place the following CSS file in /chrome/css/ and then include it in my_userChrome.css with @import "./css/additional_menubar_tabstoolbar_adjustments_128.css";.

additional_menubar_tabstoolbar_adjustments_128.css (CSS ver: 4.7.3; Firefox ver: 118–132)
#main-window #toolbar-menubar:is([autohide="false"], :not([inactive="true"])) ~ #TabsToolbar,
#main-window:not([tabsintitlebar]) #TabsToolbar {
	padding-inline-start: 0 !important;
}
#main-window[tabsintitlebar]:is([sizemode="maximized"], :not([sizemode="maximized"])):is([uidensity=compact], :not([uidensity=compact])) #toolbar-menubar[autohide="true"][inactive="true"] ~ #TabsToolbar {
	padding-inline-start: calc(var(--appbutton_in_titlebar) - 0.5rem) !important;
}
#main-window[tabsintitlebar]:is([sizemode="fullscreen"], [sizemode="maximized"]) #toolbar-menubar:is([autohide="true"]:not([inactive="true"]), [autohide="false"]) {
	padding-top: 0.25rem !important;
}
#main-window[tabsintitlebar] #toolbar-menubar:is([autohide="true"]:not([inactive="true"]), [autohide="false"]) ~ #TabsToolbar {
	margin-left: -0.064rem;
}
#main-window[tabsintitlebar]:is([sizemode="fullscreen"], [sizemode="maximized"]) #toolbar-menubar:is([autohide="true"][inactive="true"], [autohide="false"]) ~ #TabsToolbar,
#main-window[tabsintitlebar]:not([sizemode="fullscreen"], [sizemode="maximized"]) #toolbar-menubar[autohide="false"] ~ #TabsToolbar {
	margin-top: -0.15rem !important;
}
#main-window[tabsintitlebar]:is([sizemode="fullscreen"], [sizemode="maximized"]) #toolbar-menubar[autohide="true"]:not([inactive="true"]) ~ #TabsToolbar,
#main-window:not([tabsintitlebar])[sizemode="fullscreen"] #toolbar-menubar:not([inactive="true"]) ~ #TabsToolbar {
	margin-top: -0.25rem !important;
}
#main-window[tabsintitlebar]:not([sizemode="fullscreen"], [sizemode="maximized"]) #toolbar-menubar[autohide="true"][inactive="true"] ~ #TabsToolbar {
	margin-top: -0.2rem !important;
}
#main-window[tabsintitlebar]:not([sizemode="fullscreen"], [sizemode="maximized"]) #toolbar-menubar:is([autohide="true"]:not([inactive="true"]), [autohide="false"]) {
	padding-top: 0.2rem !important;
}
#main-window[tabsintitlebar]:not([sizemode="fullscreen"], [sizemode="maximized"]) #toolbar-menubar[autohide="true"]:not([inactive="true"]) ~ #TabsToolbar,
#main-window:not([tabsintitlebar])[sizemode="fullscreen"] #toolbar-menubar[inactive="true"] ~ #TabsToolbar {
	margin-top: -0.3rem !important;
}
#main-window:not([tabsintitlebar]):not([sizemode="fullscreen"]) #toolbar-menubar {
	padding: 0.15rem 0 0.2rem 0.2rem !important;
}
#main-window:not([tabsintitlebar])[sizemode="fullscreen"] #toolbar-menubar {
	padding: 0.15rem 0 0 0.2rem !important;
}
#main-window:not([tabsintitlebar])[sizemode="fullscreen"] #toolbar-menubar[autohide="true"]:not([inactive="true"]) ~ #TabsToolbar {
	margin-top: -0.4rem !important;
}
#main-window:not([tabsintitlebar]) #toolbar-menubar[inactive="true"] ~ #TabsToolbar {
	margin-top: -0.35rem;
}
#main-window:not([tabsintitlebar])[sizemode="fullscreen"] #toolbar-menubar[inactive="true"] ~ #TabsToolbar {
	margin-top: -0.3rem;
}
#main-window:not([tabsintitlebar]):is([sizemode="fullscreen"], [sizemode="maximized"]) #TabsToolbar {
	margin-left: -0.1rem;
}
#main-window:not([tabsintitlebar]):not([sizemode="fullscreen"], [sizemode="maximized"]) #TabsToolbar {
	margin-left: -0.08rem;
}
#main-window[sizemode="fullscreen"] #toolbar-menubar[autohide="false"] ~ #TabsToolbar .titlebar-buttonbox-container {
	visibility: collapse !important;
	display: none !important;
}

For Firefox 115, first replace the CSS file with the same filename as below under /chrome/css/locationbar/ with the one as follows—

locationbar/ac_popup_megabar_title_and_url_two_lines.css (CSS ver: 4.4.2; Firefox ver: 115)
/* Firefox userChrome.css tweaks ********************************************************/
/* Github: https://github.com/aris-t2/customcssforfx ************************************/
/****************************************************************************************/


/* based on [wrap]-code from default browser.css */

#urlbar .urlbarView-results > .urlbarView-row > .urlbarView-row-inner {
  flex-wrap: wrap !important;
}
#urlbar .urlbarView-results > .urlbarView-row > .urlbarView-row-inner > .urlbarView-no-wrap {
  max-width: 100% !important;
  flex-basis: 100% !important;
}
#urlbar .urlbarView-results > .urlbarView-row[has-url] > .urlbarView-row-inner > .urlbarView-url {
  margin-top: 2px !important;
  padding-inline-start: calc(6px + 1px + 16px) !important;
}

#urlbar[actionoverride] .urlbarView-results > .urlbarView-row[has-url] > .urlbarView-row-inner > .urlbarView-no-wrap > .urlbarView-title-separator,
#urlbar .urlbarView-results > .urlbarView-row[has-url]:not([type$=tab]) > .urlbarView-row-inner > .urlbarView-no-wrap > .urlbarView-title-separator {
  display: none !important;
}

/* support for heuristic items: search, remote tab, switch to tab */
#urlbar .urlbarView-results > .urlbarView-row:is([type=search],[type=switchtab]) > .urlbarView-row-inner > .urlbarView-no-wrap > .urlbarView-title-separator,
#urlbar .urlbarView-results > .urlbarView-row:not([has-url]) > .urlbarView-row-inner > .urlbarView-no-wrap > .urlbarView-title-separator {
  display: none !important;
}

#urlbar .urlbarView-results > .urlbarView-row:is([type=search],[type=switchtab]) > .urlbarView-row-inner > .urlbarView-no-wrap > .urlbarView-action,
#urlbar .urlbarView-results > .urlbarView-row:not([has-url]) > .urlbarView-row-inner > .urlbarView-no-wrap > .urlbarView-action,
#urlbar .urlbarView-results > .urlbarView-row:is([type=remotetab]):not([selected]):not(:hover) > .urlbarView-row-inner > .urlbarView-no-wrap > .urlbarView-action {
  display: block !important;
  position: absolute !important;
}

#urlbar .urlbarView-results > .urlbarView-row:is([type=search],[type=switchtab]) > .urlbarView-row-inner > .urlbarView-no-wrap > .urlbarView-action,
#urlbar .urlbarView-results > .urlbarView-row:not([has-url]) > .urlbarView-row-inner > .urlbarView-no-wrap > .urlbarView-action {
  padding-inline-start: 24px !important;
}

#urlbar .urlbarView-results > .urlbarView-row:is([type=remotetab]):not([selected]):not(:hover) > .urlbarView-row-inner > .urlbarView-no-wrap > .urlbarView-action {
  margin-bottom: -47px !important;
  padding-inline-start: 16px !important;
}

#urlbar .urlbarView-results > .urlbarView-row[type=switchtab] > .urlbarView-row-inner > .urlbarView-no-wrap > .urlbarView-action {
  margin-bottom: -47px !important;
  appearance: none !important;
  background: unset !important;
  padding-inline-start: 15px !important;
}

#urlbar .urlbarView-results > .urlbarView-row[type=remotetab] > .urlbarView-row-inner > .urlbarView-no-wrap > .urlbarView-action {
  appearance: none !important;
  background: unset !important;
}

#urlbar .urlbarView-row[dynamicType=onboardTabToSearch] > .urlbarView-row-inner {
  min-height: unset !important;
}

.urlbarView-row[dynamicType=onboardTabToSearch] > .urlbarView-row-inner > .urlbarView-no-wrap > .urlbarView-favicon {
  min-width: unset !important;
  height: unset !important;
}

.urlbarView-dynamic-onboardTabToSearch-description {
  display: none !important;
}

#urlbar .urlbarView-results > .urlbarView-row[has-url]:is([type=remotetab],[sponsored],[type=switchtab],[type=search]) > .urlbarView-row-inner > .urlbarView-url {
  display: none !important;
}

.urlbarView-row[rich-suggestion] > .urlbarView-row-inner > .urlbarView-favicon {
  width: 16px !important;
  height: 16px !important;
  flex-basis: 16px !important;
}

.urlbarView-row:not([rich-suggestion]),
.urlbarView-row[rich-suggestion][icon-size="16"] {
  --urlbarView-icon-margin-start: 0px !important;
  --urlbarView-icon-margin-end: 7px !important;
}

.urlbarView-row[label]::before {
  display: none !important;
}

.urlbarView-row[label] {
  margin-block-start: unset !important;
}

#urlbar .urlbarView-button-menu {
  z-index: 1 !important;
}

#urlbar .urlbarView-results > .urlbarView-row[rich-suggestion],
#urlbar .urlbarView-results > .urlbarView-row[role="presentation"]:is([type=search]) > .urlbarView-row-inner > .urlbarView-no-wrap > .urlbarView-action,
#urlbar .urlbarView-results > .urlbarView-row[role="presentation"]:not([has-url]) > .urlbarView-row-inner > .urlbarView-no-wrap > .urlbarView-action,
.urlbarView-row[role="presentation"][type="search"] .urlbarView-action {
  display: none !important;
}

And then place the following CSS file in /chrome/css/ and include it in my_userChrome.css with @import "./css/additional_menubar_tabstoolbar_adjustments_115.css";.

additional_menubar_tabstoolbar_adjustments_115.css (CSS ver: 4.7.3; Firefox ver: best for 113–117)
#urlbar[breakout] #urlbar-results .urlbarView-row-inner {
	padding-inline-start: 8px !important;
	padding-inline-end: 8px !important;
}
#main-window:is([tabsintitlebar], [customtitlebar]) #PanelUI-button,
#main-window:is([tabsintitlebar], [customtitlebar]) #PanelUI-button #PanelUI-menu-button {
	height: 2.2rem !important;
}
#main-window:is([tabsintitlebar], [customtitlebar]):not([sizemode="fullscreen"], [sizemode="maximized"]) #PanelUI-button,
#main-window:is([tabsintitlebar], [customtitlebar]):not([sizemode="fullscreen"], [sizemode="maximized"]) #PanelUI-button #PanelUI-menu-button {
	height: 2.3rem !important;
}
#main-window[tabsintitlebar]:is([sizemode="fullscreen"], [sizemode="maximized"]) #toolbar-menubar {
	margin-top: 0.25rem;
}
#main-window[tabsintitlebar]:not([sizemode="fullscreen"], [sizemode="maximized"]) #toolbar-menubar {
	margin-top: 0.3rem;
}
#main-window:not([tabsintitlebar]):not([sizemode="fullscreen"]) #toolbar-menubar {
	padding: 0.2rem 0 0.3rem 0.3rem;
}
#main-window:not([tabsintitlebar])[sizemode="fullscreen"] #toolbar-menubar {
	padding: 0.2rem 0 0 0.3rem;
	margin-bottom: -0.1rem;
}
#main-window:not([tabsintitlebar])[sizemode="fullscreen"] #toolbar-menubar[autohide="true"][inactive="true"] {
	margin-bottom: 0;
}
#main-window[tabsintitlebar][sizemode="fullscreen"] #toolbar-menubar[autohide="true"][inactive="true"],
#main-window[tabsintitlebar][sizemode="maximized"] #toolbar-menubar[autohide="false"],
#main-window[tabsintitlebar]:not([sizemode="fullscreen"], [sizemode="maximized"]) #toolbar-menubar[autohide="false"],
#main-window[tabsintitlebar]:not([sizemode="fullscreen"], [sizemode="maximized"]) #toolbar-menubar[autohide="true"][inactive="true"] {
	margin-bottom: -0.25rem;
}
#main-window[tabsintitlebar][sizemode="fullscreen"] #toolbar-menubar[autohide="true"][inactive="true"] ~ #TabsToolbar,
#main-window[tabsintitlebar][sizemode="maximized"] #toolbar-menubar[autohide="true"][inactive="true"] ~ #TabsToolbar,
#main-window[tabsintitlebar]:not([sizemode="fullscreen"], [sizemode="maximized"]) #toolbar-menubar[autohide="true"][inactive="true"] ~ #TabsToolbar,
#main-window[uidensity=compact][tabsintitlebar][sizemode="fullscreen"] #toolbar-menubar[autohide="true"][inactive="true"] ~ #TabsToolbar,
#main-window[uidensity=compact][tabsintitlebar][sizemode="maximized"] #toolbar-menubar[autohide="true"][inactive="true"] ~ #TabsToolbar,
#main-window[uidensity=compact][tabsintitlebar]:not([sizemode="fullscreen"], [sizemode="maximized"]) #toolbar-menubar[autohide="true"][inactive="true"] ~ #TabsToolbar {
	padding-inline-start: calc(var(--appbutton_in_titlebar) - 6px) !important;
}
#main-window[tabsintitlebar][sizemode="fullscreen"] #toolbar-menubar[autohide="true"]:not([inactive="true"]) {
	margin-bottom: -0.3rem;
}
#main-window[tabsintitlebar][sizemode="fullscreen"] #toolbar-menubar[autohide="false"] ~ #TabsToolbar,
#main-window[tabsintitlebar][sizemode="fullscreen"] #toolbar-menubar[autohide="true"]:not([inactive="true"]) ~ #TabsToolbar,
#main-window[tabsintitlebar][sizemode="maximized"] #toolbar-menubar[autohide="false"] ~ #TabsToolbar,
#main-window[tabsintitlebar][sizemode="maximized"] #toolbar-menubar[autohide="true"]:not([inactive="true"]) ~ #TabsToolbar,
#main-window[tabsintitlebar]:not([sizemode="fullscreen"], [sizemode="maximized"]) #toolbar-menubar[autohide="true"]:not([inactive="true"]) ~ #TabsToolbar,
#main-window[tabsintitlebar]:not([sizemode="fullscreen"], [sizemode="maximized"]) #toolbar-menubar[autohide="false"] ~ #TabsToolbar,
#main-window[uidensity=compact][tabsintitlebar][sizemode="fullscreen"] #toolbar-menubar[autohide="false"] ~ #TabsToolbar,
#main-window[uidensity=compact][tabsintitlebar][sizemode="fullscreen"] #toolbar-menubar[autohide="true"]:not([inactive="true"]) ~ #TabsToolbar,
#main-window[uidensity=compact][tabsintitlebar][sizemode="maximized"] #toolbar-menubar[autohide="false"] ~ #TabsToolbar,
#main-window[uidensity=compact][tabsintitlebar][sizemode="maximized"] #toolbar-menubar[autohide="true"]:not([inactive="true"]) ~ #TabsToolbar,
#main-window[uidensity=compact][tabsintitlebar]:not([sizemode="fullscreen"], [sizemode="maximized"]) #toolbar-menubar[autohide="true"]:not([inactive="true"]) ~ #TabsToolbar,
#main-window[uidensity=compact][tabsintitlebar]:not([sizemode="fullscreen"], [sizemode="maximized"]) #toolbar-menubar[autohide="false"] ~ #TabsToolbar {
	padding-inline-start: 0 !important;
	margin-left: -0.05rem;
}
#main-window[tabsintitlebar][sizemode="fullscreen"] #toolbar-menubar[autohide="false"] {
	margin-bottom: 0.1rem;
}
#main-window[tabsintitlebar][sizemode="maximized"] #toolbar-menubar[autohide="true"]:not([inactive="true"]) {
	margin-bottom: -0.625rem;
}
#main-window[tabsintitlebar]:not([sizemode="fullscreen"], [sizemode="maximized"]) #toolbar-menubar[autohide="true"]:not([inactive="true"]) {
	margin-bottom: -0.65rem;
}
#main-window:not([tabsintitlebar]):is([sizemode="fullscreen"], [sizemode="maximized"]) #TabsToolbar {
	padding-inline-start: 0 !important;
	margin-left: -0.1rem;
}
#main-window:not([tabsintitlebar]):not([sizemode="fullscreen"], [sizemode="maximized"]) #TabsToolbar {
	padding-inline-start: 0 !important;
	margin-left: -0.08rem;
}
#main-window[sizemode="fullscreen"] #toolbar-menubar[autohide="false"] ~ #TabsToolbar .titlebar-buttonbox-container {
	visibility: collapse !important;
	display: none !important;
}

userChrome.css & userContent.css Modifications and Additions

userChrome.css (CSS ver: 4.7.3; Firefox ver: 102–140)
/* @import "./css/buttons/icons_colorized.css"; /**/

/* @import "./css/buttons/buttons_on_navbar_classic_appearance.css"; /**/

/* @import "./css/buttons/bookmarks_menu_button_localized_label_on_bookmarks_toolbar.css"; /**/

@import "./css/buttons/back_and_forward_separated_history_popups.css"; /**/

@import "./css/appbutton/appbutton_popup_sync_hidden.css"; /**/
@import "./css/appbutton/appbutton_popup_logins_and_passwords_hidden.css"; /**/

@import "./css/appbutton/appbutton_on_navbar_start_position.css"; /**/

/* @import "./css/tabs/missing_tab_favicon_restored_default.css"; /**/

@import "./css/tabs/tab_audio_icon.css"; /**/

/* ------------------
   OTHER TAB SETTINGS
   ------------------
*/

/* @import "./css/tabs/tab_icon_colors.css"; /**/
/* @import "./css/tabs/tab_title_centered.css"; /**/
/* @import "./css/tabs/classic_squared_tabs_old_bordercolor_for_lwthemes.css"; /**/ /* has to be used along with 'classic_squared_tabs.css' */
/* @import "./css/tabs/tabs_fully_squared.css"; /**/

/* @import "./css/tabs/tabs_container_indicator_for_classic_squared_tabs.css"; /**/
/* @import "./css/tabs/tabs_active_tab_indicator_for_classic_squared_tabs.css"; /**/
/* @import "./css/tabs/newtab_tab_size_equals_tab_size.css"; /**/
/* @import "./css/tabs/newtab_button_always_visible.css"; /**/
/* @import "./css/tabs/tab_icon_inactive_tabs_lower_opacity.css"; /**/
/* @import "./css/tabs/tab_icon_unloaded_tabs_lower_opacity.css"; /**/
/* @import "./css/tabs/tab_maxwidth.css"; /**/
@import "./css/tabs/tab_titles_remove_blur.css"; /**/
/* @import "./css/tabs/alltabs_button_always_visible.css"; /**/  /* hidden, if multiple tab rows are used */
@import "./css/tabs/alltabs_button_only_visible_in_overflow_mode.css"; /**/ 
/* @import "./css/tabs/alltabs_button_always_hidden.css"; /**/ 
/* @import "./css/tabs/alltabs_button_old_icon.css"; /**/ 
/* @import "./css/tabs/pinnedtab_empty_favicon_hidden.css"; /**/

/* [!] edit target file to select which items to hide */
@import "./css/tabs/tab_context_menuitems_visibility.css"; /**/


/* ==========
   GENERAL UI
   ==========
*/

@import "./css/generalui/private_mode_indicator_hidden.css"; /**/
@import "./css/generalui/bookmark_icons_colorized.css"; /**/
/* @import "./css/generalui/searchbookmarks_hidden.css"; /**/
/* @import "./css/generalui/bookmarks_smaller_more_bookmarks_icon.css"; /**/
@import "./css/generalui/send_to_device_menuitems_hidden.css"; /**/
@import "./css/generalui/set_as_desktop_background_menuitems_hidden.css"; /**/

@import "./css/generalui/popup_compact_menus.css"; /**/
@import "./css/generalui/popup_compact_menus_squared.css"; /**/

Turn on either of these two accordingly: 

/* @import "./css/generalui/popup_menus_space_before_label.css"; /**/ /* for Fx 139 and older only */
@import "./css/generalui/popup_menus_space_before_label_fx140.css"; /**/ /* for Fx 140+ (test only) */

Then, 

@import "./css/generalui/popup_animation_fade_time_reduced.css"; /**/

@import "./css/generalui/findbar_close_at_findbars_start.css"; /**/

@import "./css/toolbars/toolbar_context_menuitems_visibility.css"; /**/

@import "./css/toolbars/menubar_in_fullscreen_mode.css"; /**/

@import "./css/toolbars/bookmarks_toolbar_in_fullscreen_mode.css"; /**/

@import "./css/locationbar/urlbar_searchmode_switcher_hidden.css"; /**/ /* currently on Nightly */

@import "./css/locationbar/starbutton_popup_preview_image_hidden.css"; /**/

@import "./css/locationbar/starbutton_alternative_icon.css"; /**/

Enable (by removing the /* at the beginning of the line) the second line (line 1228) and disable (by adding a /* to the beginning of the line) the first (line 1227) if the version of Firefox installed is 132 or below; if not (the version of Firefox installed is 133 or above), keep the original settings intact— 

@import "./css/locationbar/megabar_expanding_breakout_disabled.css"; /**/
/* @import "./css/locationbar/megabar_expanding_breakout_disabled_fx128-132.css"; /**/

And lastly, 

/* @import "./css/locationbar/urlbar_background_old_colors.css"; /**/

/* ---------------------------------------
   Megabar: popup content order/appearance
   ---------------------------------------
   
   [!] only use one option at a time
*/

/* @import "./css/locationbar/ac_popup_megabar_title_and_url_50percent_width.css"; /**/
/* @import "./css/locationbar/ac_popup_megabar_url_and_title_50percent_width.css"; /**/
@import "./css/locationbar/ac_popup_megabar_title_and_url_two_lines.css"; /**/
/* @import "./css/locationbar/ac_popup_megabar_url_only.css"; /**/


/* --------------------------------------------
   MEGABAR: result menuitem settings/appearance
   --------------------------------------------
*/

@import "./css/locationbar/ac_popup_megabar_compact_results.css"; /**/
@import "./css/locationbar/ac_popup_megabar_searchwith_and_visit_items_hidden.css"; /**/
@import "./css/locationbar/ac_popup_megabar_suggest_label_hidden.css"; /**/
@import "./css/locationbar/ac_popup_megabar_search_engines_hidden.css"; /**/
userContent.css (CSS ver: 4.7.3; Firefox ver: 102–140)
/* @import "./css/aboutaddons/addons_manager_alternative_appearance.css"; /**/

@import "./css/aboutaddons/addonlists_hide_report_button.css"; /**/

@import "./css/aboutaddons/addonlists_hide_recommended_icon.css"; /**/
/* @import "./css/aboutaddons/addonlists_hide_allowed_in_private_windows_icon.css"; /**/
@import "./css/aboutaddons/recentupdates_category_always_visible.css"; /**/
@import "./css/aboutaddons/availableupdates_category_always_visible.css"; /**/
@import "./css/aboutaddons/availableupdates_category_last_category.css"; /**/
@import "./css/aboutaddons/recommendations_category_always_hidden.css"; /**/
@import "./css/aboutaddons/addons_support_always_hidden.css"; /**/

/* @import "./css/aboutpreferences/preferences_alternative_appearance.css"; /**/

@import "./css/aboutpreferences/category_morefrommozilla_hidden.css"; /**/
@import "./css/aboutpreferences/category_nightlyexperiments_hidden.css"; /**/
@import "./css/aboutpreferences/sync_category_always_hidden.css"; /**/
@import "./css/aboutpreferences/firefox_support_always_hidden.css"; /**/

/* @import "./css/aboutlogins/aboutlogins_alternative_appearance.css"; /**/

And lastly, append the following lines to the end of the file. 

@import "./css/generalui/popup_compact_menus.css";
@import "./css/generalui/popup_compact_menus_squared.css";
@import "./css/generalui/popup_menus_space_before_label.css";
@import "./config/general_variables.css";
@import "./config/color_variables.css";
@import "./css/tabs/tab_context_menuitems_visibility.css";
@import "./css/generalui/send_to_device_menuitems_hidden.css";
@import "./css/generalui/set_as_desktop_background_menuitems_hidden.css";
@import "./css/generalui/findbar_close_at_findbars_start.css";
@import "./css/toolbars/toolbar_context_menuitems_visibility.css";
@import "./css/toolbars/general_toolbar_colors.css";
my_userChrome.css (CSS ver: 4.7.3; Firefox ver: 102–140)
/* @import "./css/additional_menubar_tabstoolbar_adjustments_115.css"; /* Apply one of these three according to your Firefox version. */
/* @import "./css/additional_menubar_tabstoolbar_adjustments_128.css"; /* Apply one of these three according to your Firefox version. */
/* @import "./css/additional_menubar_tabstoolbar_adjustments_133.css"; /* Apply one of these three according to your Firefox version. */

:root {
	--urlbarView-small-font-size: 1em !important;
}

menuseparator,
toolbarseparator {
	padding: 0.06em 0 !important;
	margin-left: 0 !important;
	margin-right: 0 !important;
}

menuitem:not([acceltext]) {
	padding-inline-end: 16px !important;
}

toolbarbutton#appMenu-empty-profiles-button::before {
	content: "　    ";
	display: inline !important;
}

:is(menu, menuitem) > image.menu-icon {
	margin-right: 6px !important;
}

.toolbar-menupopup :is(menu, menuitem) {
	min-height: 24px;
}

#context_moveTabToNewGroup,
#context_moveTabToGroup,
#context_ungroupTab,
#toolbar-context-toggle-vertical-tabs,
#context_MoveTabToOtherWorkspace,
#context_splittabs,
#context_split_fixedtab,
#context_splitView,
[id="placesContext_open:newcontainertab"],
#splitViewTabContextMenuClose,
#context_toggleToPrivateContainer,
#open_in_private_container,
#bsb-context-add,
#bsb-context-link-add,
#context-searchselect-private,
#alltabs-button,
#appMenu-fxa-separator,
#appMenu-new-tab-button2,
#appMenu-translate-button,
#appMenu-whatsnew-button,
#appMenu-zoom-controls,
#context_closeTab,
#context_reopenInContainer,
#context_shareTabURL,
#context-back,
#context-bookmarklink,
#context-bookmarkpage,
#context-forward,
#context-inspect-a11y,
#context-media-pause,
#context-media-play,
#context-openlink,
#context-openlinkincontainertab,
#context-openlinkincurrent,
#context-openlinkinusercontext-menu,
#context-pocket,
#context-print-selection,
#context-reload,
#context-savelinktopocket,
#context-savepage,
#context-sendaudio,
#context-sendimage,
#context-sendlinktodevice,
#context-sendpagetodevice,
#context-sendvideo,
#context-sep-navigation,
#context-sep-screenshots,
#context-sep-selectall,
#context-sep-sendlinktodevice,
#context-sep-setbackground,
#context-sep-sharing,
#context-stop,
#context-take-screenshot,
#context-translate-selection,
#context-viewsource,
#customize-context-reportExtension,
#import-button,
#placesContext_sortBy,
#places-sortby-name,
#tabContextMenu #context_closeTabOptions,
#tabContextMenu #context_duplicateTabs,
#tabContextMenu #context_openANewTab,
#tabContextMenu .share-tab-url-item,
#toggle_PersonalToolbar,
#toolbar-context-menu #toggle_toolbar-menubar,
#tabContextMenu #toggle_toolbar-menubar,
#toolbar-context-autohide-downloads-button,
#toolbar-context-bookmarkSelectedTab,
#toolbar-context-bookmarkSelectedTabs,
#toolbar-context-menu .customize-context-reportExtension,
#toolbar-context-openANewTab,
#toolbar-context-reloadSelectedTab,
#toolbar-context-reloadSelectedTabs,
#_531906d3-e22f-4a6c-a102-8057b88a1a63_-menuitem-_save-selected-links,
.customize-context-moveToPanel,
[id^="_81231243-7deb-4d81-bc8a-4a7f3eb62144_-menuitem-_note"],
[id^="maxurl_qsniyg-menuitem"],
[id^="opencc_extension_tnychn-menuitem"],
#tabContextMenu [id^="_531906d3-e22f-4a6c-a102-8057b88a1a63_-menuitem"],
#context_split_fixedtab + menuseparator,
#contentAreaContextMenu #context-print-selection + menuseparator,
#context-bookmarkpage + menuseparator,
#context-selectall + menuseparator,
#context-sendlinktodevice + menuseparator,
#context-sendlinktodevice-popup + menuseparator,
#context-setDesktopBackground + menuseparator,
#context-take-screenshot + menuseparator,
#context-translate-selection + menuseparator,
#context-viewimagedesc + menuseparator,
#context_ungroupTab + menuseparator,
#frame + menuseparator,
#tabContextMenu #context_duplicateTabs + menuseparator,
#tabContextMenu #context_openANewTab + menuseparator,
#tabContextMenu #context_pinTab + menuseparator,
#tabContextMenu #context_selectAllTabs + menuseparator,
#tabContextMenu .share-tab-url-item + menuseparator,
#toggle_PersonalToolbar + menuseparator,
#toggle_toolbar-menubar + menuseparator,
#toolbar-context-menu .customize-context-reportExtension + menuseparator,
#toolbar-context-openANewTab + menuseparator,
#toolbar-context-undoCloseTab + menuseparator,
#context-searchselect-private + menuseparator {
	visibility: collapse !important;
	display: none !important;
}

.urlbarView-url[overflow] {
	mask-image: none !important;
}
span[class="urlbarView-url"] {
	margin-right: 0.81em !important;
}

#identity-icon-box {
	background-color: rgba(255,255,255,0) !important;
}

#forecastfox_s3_fix_version-BAP {
	padding-right: 0.36em !important;
}

#context-openlinkintab {
	-webkit-box-ordinal-group: -12 !important;
	-moz-box-ordinal-group: -12 !important;
	-box-ordinal-group: -12 !important;
	order: -12 !important;
}
[id^="copyplaintext_eros_man-menuitem"] {
	-webkit-box-ordinal-group: -10 !important;
	-moz-box-ordinal-group: -10 !important;
	-box-ordinal-group: -10 !important;
	order: -10 !important;
}
[id^="_6779b4de-d680-462a-8038-43079e365983_-menuitem"] {
	-webkit-box-ordinal-group: -8 !important;
	-moz-box-ordinal-group: -8 !important;
	-box-ordinal-group: -8 !important;
	order: -8 !important;
}
[id^="highlightedtexttofile_bobbyrne01_org-menuitem"] {
	-webkit-box-ordinal-group: -7 !important;
	-moz-box-ordinal-group: -7 !important;
	-box-ordinal-group: -7 !important;
	order: -7 !important;
}
#contentAreaContextMenu #context-searchselect {
	-webkit-box-ordinal-group: -6 !important;
	-moz-box-ordinal-group: -6 !important;
	-box-ordinal-group: -6 !important;
	order: -6 !important;
}
[id^="context-wiktionary_addons_mozilla_org-menuitem"] {
	-webkit-box-ordinal-group: -5 !important;
	-moz-box-ordinal-group: -5 !important;
	-box-ordinal-group: -5 !important;
	order: -5 !important;
}
[id^="_e040cf12-0537-4265-8ce0-ee195356ed60_-menuitem"] {
	-webkit-box-ordinal-group: -4 !important;
	-moz-box-ordinal-group: -4 !important;
	-box-ordinal-group: -4 !important;
	order: -4 !important;
}

[id^="popupwindow_ettoolong-menuitem"] label {
	-webkit-box-ordinal-group: 1 !important;
	-moz-box-ordinal-group: 1 !important;
	-box-ordinal-group: 1 !important;
	order: 1 !important;
	visibility: collapse !important;
	display: none !important;
}
[id^="popupwindow_ettoolong-menuitem"]::after {
	content: "Pop Out / Remerge";
	display: -moz-box;
}

[id^="copytabtitleurl_bugbugnow_net-menuitem-_menu"] label {
	visibility: collapse !important;
	display: none !important;
}
#copytabtitleurl_bugbugnow_net-menuitem-_menu0::after {
	content: "Both";
	display: -moz-box;
	padding-inline-end: 0.8rem;
}
#copytabtitleurl_bugbugnow_net-menuitem-_menu1::after {
	content: "Title(s) only";
	display: -moz-box;
	padding-inline-end: 0.8rem;
}
#copytabtitleurl_bugbugnow_net-menuitem-_menu2::after {
	content: "URL(s) only";
	display: -moz-box;
	padding-inline-end: 0.8rem;
}

#context-openlinkprivate {
	-webkit-box-ordinal-group: -11 !important;
	-moz-box-ordinal-group: -11 !important;
	-box-ordinal-group: -11 !important;
	order: -11 !important;
}
#context-openlinkprivate label {
	visibility: collapse !important;
	display: none !important;
}
#context-openlinkprivate::after {
	content: "Open in Private Window";
	display: -moz-box;
}

#context-stripOnShareLink label {
	visibility: collapse !important;
	display: none !important;
}
#context-stripOnShareLink::after {
	content: "Copy Clean Link (U)";
	display: -moz-box;
}

#strip-on-share label {
	visibility: collapse !important;
	display: none !important;
}
#strip-on-share::after {
	content: "Copy Clean (N)";
	display: -moz-box;
}

#_save-url-to-wayback-machine-firefox-addon-menuitem-_archive-url-wayback label {
	visibility: collapse !important;
	display: none !important;
}
#_save-url-to-wayback-machine-firefox-addon-menuitem-_archive-url-wayback::after {
	content: "Wayback Machine";
	display: -moz-box;
	padding-inline-end: 0.8rem;
}
#_save-url-to-wayback-machine-firefox-addon-menuitem-_archive-url-archive label {
	visibility: collapse !important;
	display: none !important;
}
#_save-url-to-wayback-machine-firefox-addon-menuitem-_archive-url-archive::after {
	content: "archive.today";
	display: -moz-box;
	padding-inline-end: 0.8rem;
}
#_save-url-to-wayback-machine-firefox-addon-menuitem-_archive-url-ghost-archive label {
	visibility: collapse !important;
	display: none !important;
}
#_save-url-to-wayback-machine-firefox-addon-menuitem-_archive-url-ghost-archive::after {
	content: "Ghost Archive";
	display: -moz-box;
	padding-inline-end: 0.8rem;
}
#_save-url-to-wayback-machine-firefox-addon-menuitem-_archive-url-both label {
	visibility: collapse !important;
	display: none !important;
}
#_save-url-to-wayback-machine-firefox-addon-menuitem-_archive-url-both::after {
	content: "All above";
	display: -moz-box;
	padding-inline-end: 0.8rem;
}
#_save-url-to-wayback-machine-firefox-addon-menuitem-_archive-image-url-wayback label {
	visibility: collapse !important;
	display: none !important;
}
#_save-url-to-wayback-machine-firefox-addon-menuitem-_archive-image-url-wayback::after {
	content: "Image > Wayback Machine";
	display: -moz-box;
	padding-inline-end: 0.8rem;
}
#_save-url-to-wayback-machine-firefox-addon-menuitem-_archive-link-url-wayback label {
	visibility: collapse !important;
	display: none !important;
}
#_save-url-to-wayback-machine-firefox-addon-menuitem-_archive-link-url-wayback::after {
	content: "Link > Wayback Machine";
	display: -moz-box;
	padding-inline-end: 0.8rem;
}
#_save-url-to-wayback-machine-firefox-addon-menuitem-_archive-image-url-archive label {
	visibility: collapse !important;
	display: none !important;
}
#_save-url-to-wayback-machine-firefox-addon-menuitem-_archive-image-url-archive::after {
	content: "Image > archive.today";
	display: -moz-box;
	padding-inline-end: 0.8rem;
}
#_save-url-to-wayback-machine-firefox-addon-menuitem-_archive-link-url-archive label {
	visibility: collapse !important;
	display: none !important;
}
#_save-url-to-wayback-machine-firefox-addon-menuitem-_archive-link-url-archive::after {
	content: "Link > archive.today";
	display: -moz-box;
	padding-inline-end: 0.8rem;
}
#_save-url-to-wayback-machine-firefox-addon-menuitem-_archive-image-url-ghost-archive label {
	visibility: collapse !important;
	display: none !important;
}
#_save-url-to-wayback-machine-firefox-addon-menuitem-_archive-image-url-ghost-archive::after {
	content: "Image > Ghost Archive";
	display: -moz-box;
	padding-inline-end: 0.8rem;
}
#_save-url-to-wayback-machine-firefox-addon-menuitem-_archive-link-url-ghost-archive label {
	visibility: collapse !important;
	display: none !important;
}
#_save-url-to-wayback-machine-firefox-addon-menuitem-_archive-link-url-ghost-archive::after {
	content: "Link > Ghost Archive";
	display: -moz-box;
	padding-inline-end: 0.8rem;
}
#_save-url-to-wayback-machine-firefox-addon-menuitem-_archive-image-url-both label {
	visibility: collapse !important;
	display: none !important;
}
#_save-url-to-wayback-machine-firefox-addon-menuitem-_archive-image-url-both::after {
	content: "Image > All above";
	display: -moz-box;
	padding-inline-end: 0.8rem;
}
#_save-url-to-wayback-machine-firefox-addon-menuitem-_archive-link-url-both label {
	visibility: collapse !important;
	display: none !important;
}
#_save-url-to-wayback-machine-firefox-addon-menuitem-_archive-link-url-both::after {
	content: "Link > All above";
	display: -moz-box;
	padding-inline-end: 0.8rem;
}

[id^="_ee9507aa-5b54-4548-a8b1-f1840b6d5c26_"] label {
	visibility: collapse !important;
	display: none !important;
}
[id^="_ee9507aa-5b54-4548-a8b1-f1840b6d5c26_"]::after {
	content: "Background Image";
	display: -moz-box;
	padding-inline-end: 0.8rem;
}

If in-browser profile management and switching is completely not desired, then remove lines from 20 to 24, and add menu#profiles-menu, and toolbarbutton[id$="profiles-button"], at the beginning of the mass hiding block, so the beginning part of this file shall look like as follows:

my_userChrome.css (EXORDIVM ALTERNATVM; CSS ver: 4.7.3)
/* @import "./css/additional_menubar_tabstoolbar_adjustments_115.css"; /* Apply one of these three according to your Firefox version. */
/* @import "./css/additional_menubar_tabstoolbar_adjustments_128.css"; /* Apply one of these three according to your Firefox version. */
/* @import "./css/additional_menubar_tabstoolbar_adjustments_133.css"; /* Apply one of these three according to your Firefox version. */

:root {
	--urlbarView-small-font-size: 1em !important;
}

menuseparator,
toolbarseparator {
	padding: 0.06em 0 !important;
	margin-left: 0 !important;
	margin-right: 0 !important;
}

menuitem:not([acceltext]) {
	padding-inline-end: 16px !important;
}

:is(menu, menuitem) > image.menu-icon {
	margin-right: 6px !important;
}

.toolbar-menupopup :is(menu, menuitem) {
	min-height: 24px;
}

menu#profiles-menu,
toolbarbutton[id$="profiles-button"],
#context_moveTabToNewGroup,
#context_moveTabToGroup,
#context_ungroupTab,
#toolbar-context-toggle-vertical-tabs,
my_userContent.css (CSS ver: 4.7.3; Firefox ver: 102–140)
panel-item + hr {
	visibility: collapse !important;
	display: none !important;
}
menuseparator,
toolbarseparator {
	padding: 0.06em 0 !important;
	margin-left: 0 !important;
	margin-right: 0 !important;
}

Straighten Up the Rounded Corners

To ‘unround’ all the utter ugly rounded corners, add the following lines to my_userChrome.css—

my_userChrome.css (ADDENDVM)
* {
	&, &::before, &::after {
		border-radius: 0 !important;
	}
}

Then these lines below to my_userContent.css—

my_userContent.css (ADDENDVM)
#preferences-root *,
body:has(> div#full) *,
body:has(> div.container) *,
body.aboutPageWideContainer *,
body:has(> div#toolbar) *,
[force-theme] *,
#contentAreaDownloadsView *,
body#lic-info *,
body#body *,
body:has( fxaccounts-button) *,
body:has(> div.ancillary) *,
body.wide-container *,
body:has(> *[data-l10n-id^="about"]) *,
body:has(> p[id^="UI_"]) *,
body:has(> div#app) *,
body:has(> div.container) *,
[focuseddatatype] *,
body.theme-body *,
body:has(> p.showNormal) *,
#moz-toggle-button {
	&, &::before, &::after {
		border-radius: 0 !important;
	}
}

And if you would also like to remove those tiny little rounded corners around the tabs (which I personally do not dislike), just go back to userChrome.css and enable (by removing the /* at the beginning of the line) the following option—

userChrome.css (CSS ver: 4.7.3; OPTIONAL MODIFICATION)
@import "./css/tabs/tabs_fully_squared.css"; /**/

Furthermore, if you, like me, really hate all those rounded corners all around the web, just try out this absolutely wonderful userstyle with Stylus; it will help you to eradicate all of them, once for all.


检索自“https://wiki.tsetien.com/index.php?title=Firefox_CSS_Modification&oldid=2976”