Is a left-side tab close button in Firefox possible?

Question

The tab close button in Firefox is on the right side. But that feels weird in macOS, where all close buttons are on the left side. Is it possible to move it to the left side, possibly with an extension or a configuration setting?

Answer 1

As luck would have it, the Firefox UI can be customized in ~/Library/Application Support/Firefox/Profiles/<PROFILE_ID>/chrome/userChrome.css. Add the following to that file (valid for Firefox v98):

@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;
}

Also, userChrome.css is disabled by default in Firefox 69+, so go to about:config and set toolkit.legacyUserProfileCustomizations.stylesheets to true

Thanks to https://support.mozilla.org/en-US/questions/1157451 for inspiration. I've also added a Gist with this info.