NOTE 2021-06-11: This extension is currently unmaintained. I abandoned this when I ended up using a Firefox userChrome.css addition (updated for Firefox 89):

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: 24px;
  top: 7px;
  padding: 0px 4px;
  border-radius: 7px;

  opacity: .7;
  background: #ff0c;

  font-weight: bold;
  font-size: 80%;
}

/* show vertical lines between tabs */
tab:not(:first-child) {
  border-left: 1px solid #0004 !important;
}

• Pros: very robust; easy to customize styles
• Cons: harded to install than an extension. See instructions e.g. here.

tab-numbering

A browser extension for add numbering to tab titles. Uses WebExtensions API, and thus works on at least Chrome and Firefox.

Chrome:

Firefox:

This extension writes the tab number to the first eight tabs, the ones accessible with ctrl/cmd + number.

How to install

• For Firefox: install from Mozilla Add-Ons.
• For Chrome:
  1. Clone this repository to your machine
  2. Open chrome://extensions/
  3. Check Developer mode
  4. Click Load unpacked extension...
  5. Select the directory you cloned the repository into.

Known issues

• does not add numbers to pinned tabs, internal error pages, "new tab" pages or other special tabs

• does not keep in sync when dragging tabs to/from another window

• will mess up with titles already starting with the characters ¹...⁸