The Mac Firebird theme is still not compatible with Tabbrowser Extentions. I’ve contacted the TBE author about compatibility but have not received a reply. If any of you would like to let the author know that you’d like TBE to work with Mac Firebird, you can post here: http://white.sakura.ne.jp/~piro/xul/forum/
Update January 23: Apparently there is a patch on Piro’s site that makes TBE work with Mac Firebird. I can’t confirm this and I probably won’t have time to test it this weekend. I’d be grateful if someone could download the patch, try it out and write out simple instructions that I can post here. Thanks!