I LOVE my Firefox, and would never switch back to IE, but I had problems with Thunderbird. When I would get a spam with a 1969 date (why the hell would it have that date I don't know) it would freeze up and not download off the server. If they could get that fixed I'd love to dump my web based mail.
BTW, check out the firefox extensions.... that's when it really starts to rock!
http://texturizer.net/firefox/extensions/
adblock, clone window, and IE View are my favorites.