custom_default_tab_favicons.css

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

/* Icon files not included. Save the icon files to same folder where userChrome.css is */

 /* Default tab favicon, the globe kind of thing */
.tab-icon-image:not([src]){
  content: url("icon.png"); /* filename for icon to load */
}

/* The Firefox brand icon seen on newtab page etc. */
.tab-icon-image[src="chrome://branding/content/icon32.png"]{
  content: url("icon.png"); /* filename for icon to load */
}