Run a full AV scan and also use Chrome or IE (never thought I'd recommend it ever) instead.
Reinstall of Firefox might be a good idea as well but ensure you uninstall it first. As for your bookmarks and stuff...well, you could import them into another browser
Do a spyware scan (program like SpyBot Search and Destroy [Download]). Also run McAfee Stinger [Download] (run this first actually). Then run CCleaner [Download] (most likely will get rid of any saved passwords you have in FireFox, believe they now have an option though to not erase those cookies).
I use facebook heaps, and I haven't seen anything remotely malware, only issue I have is privacy. Not saying "just because I don't, everybody does", I've just never heard of Facebook being a vector for attacks.
OP, also give MBAM a shot, I use it as an on-demand scanner and works fairly well when I diagnose other people's malware problems.
The only source of virus' on Facebook is if somebody is idiotic enough to click a link posted by a friend (well, an application posting it), saying something like, "OMG!1 ELEFANT WIF 6 LEGZ! CLICK HEAR FOR MOAR..."
This is what happens when you click on a link from a message from your friend that say "OMG CHECK THIS OUT IT'S SO INSANE!"...
I actually made a video about this awhile ago: http://www.youtube.com/watch?v=PGuZxQ-dJwE
Ah, after watching this video again it made me remember the old laptop