mobile-config-firefox/chrome/numbered_tabs.css

/* Source file https://github.com/MrOtherGuy/firefox-csshacks/tree/master/chrome/numbered_tabs.css made available under Mozilla Public License v. 2.0
See the above repository for updates as well as full license text. */

/* Show a number before tab text*/

.tabbrowser-tab:first-child{ counter-reset: nth-tab 0 } /* Change to -1 for 0-indexing */
.tab-text::before{ content: counter(nth-tab) " "; counter-increment: nth-tab }