You can fix the tab icon using userChrome.css. Put the icon image (google it) in the directory with your userChrome.css file. There may be a better CSS selector combo, but I found this through some trial and error and it seems to work fine.
/* restore old twitter logo in tabs */ .tabbrowser-tab[label*="/ Twitter"] .tab-content .tab-icon-image { content: url("Logo_of_Twitter.svg") !important; }
