My custom `profile_root/chrome/userChrome.css` file for tab numbering in Firefox

userChrome.css

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

tab {
  counter-increment: tabnumber;
}

.tabbrowser-tab:not([pinned]):not([selected]):before {
  background: none !important;
  color: black !important;
  content: "\A0\A0" counter(tabnumber) !important;
  font-size: 9px !important;
  font-weight: bold !important;
  margin-right: -1em !important;
  opacity: 0.5 !important;
  padding-top: 18px !important;
  position: absolute !important;
  z-index: 65535 !important;
}

.tabbrowser-tab::after, .tabbrowser-tab::before {
  border: none !important;
}

#PopupAutoCompleteRichResult[type='autocomplete-richlistbox'] {
  opacity: 0.9 !important;
}

#PopupAutoCompleteRichResult[type='autocomplete-richlistbox']:hover {
  opacity: 0.4 !important;
}