mobile-config-firefox/chrome/more_visible_tab_icon.css

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

/* Makes black favicons more visible on dark background, contrast will be lowered though */
.tab-icon-image{ filter: invert(40%) contrast(250%) saturate(250%) !important; }