mobile-config-firefox/chrome/tabs_fill_available_width.css

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

/* Why 100vw? Tab closing requires width animation to end and "none" can't be animated */
.tabbrowser-tab[fadein]{ max-width: 100vw !important }