Tab close button on the left in Firefox Quantum

Tab close button on the left in Firefox Quantum.md

Go to about:config and set toolkit.legacyUserProfileCustomizations.stylesheets to true

Add userChrome.css to ~/Library/Application Support/Firefox/Profiles/<PROFILE_ID>/chrome/userChrome.css

Ref:

• https://developer.mozilla.org/en-US/docs/Tools/Browser_Toolbox
• https://support.mozilla.org/en-US/questions/1157451
• https://www.ghacks.net/2019/05/24/firefox-69-userchrome-css-and-usercontent-css-disabled-by-default/
• https://superuser.com/questions/1495834/is-a-left-side-tab-close-button-in-firefox-possible

History:

• 2022-03-01: Updated for Firefox v98

userChrome.css

@namespace url(http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul);

.tabbrowser-tab .tab-throbber,
.tabbrowser-tab .tab-icon-image,
.tabbrowser-tab .tab-sharing-icon-overlay,
.tabbrowser-tab .tab-icon-overlay,
.tabbrowser-tab .tab-label-container,
.tabbrowser-tab .tab-icon-sound {
  -moz-box-ordinal-group: 2 !important;
}
.tabbrowser-tab .tab-label-container {
  margin-left: 2px;
}
.tabbrowser-tab .tab-icon-image {
  margin-left: 3px;
}
.tabbrowser-tab .tab-close-button {
  margin-left: -5px !important;
  margin-right: 2.5px !important;
}
.tabbrowser-tab:not([pinned="true"]):hover .tab-close-button {
  display: -moz-box !important;
}
.tabbrowser-tab:not([pinned="true"]):hover .tab-icon-stack,
.tabbrowser-tab:not([pinned="true"]):hover .tab-throbber,
.tab-close-button {
  display: none;
}

.tabbrowser-tab .tab-close-button {
   opacity: 0;
   margin-left: -4.5px !important;
   margin-right: 2px !important;
 }
 .tabbrowser-tab:not(:hover) .tab-close-button {
   display:none;
 }
 .tabbrowser-tab:not([pinned="true"]):hover .tab-close-button {
   opacity: 1;
   margin-inline-end: 0;
   order:-1 !important;
   display:unset !important;

}
.tabbrowser-tab:not([pinned="true"]):hover .tab-icon-stack {
-moz-box-ordinal-group: 99999 !important;
}
.tabbrowser-tab:not([pinned="true"]):hover .tab-icon-stack:not([indicator-replaces-favicon]) {
display: none;
}