Firefox numbered tabs

description.txt

Source - https://github.com/tuomassalo/tab-numbering/blob/master/README.md
Add to /home/<user>/.mozilla/firefox/<profile>/chrome/userChrome.css
Enable toolkit.legacyUserProfileCustomizations.stylesheets in about:config

userChrome.css

tabs {
  counter-reset: tab-counter;
}

tab:nth-child(1) .tab-content::before,
tab:nth-child(2) .tab-content::before,
tab:nth-child(3) .tab-content::before,
tab:nth-child(4) .tab-content::before,
tab:nth-child(5) .tab-content::before,
tab:nth-child(6) .tab-content::before,
tab:nth-child(7) .tab-content::before,
tab:nth-child(8) .tab-content::before {
  counter-increment: tab-counter;
  content: counter(tab-counter) "";

  position: absolute;
  left: 22px;
  top: 6px;
  padding: 0px 4px;
  border-radius: 7px;
  
  opacity: .7;
  font-weight: bold;
  font-size: 70%;
}