Firefox bugs?

I just got a new macbook and downloaded firefox. after a while, it just doesnt seem to work. pages load funny, and email is screwed up. If I use safari, its fine, but for a silly reason I have to have two browsers operating at the same time. (I used two different yahoo addresses for work and can’t open two accounts in one browser.) Anyone have any idea what’s going on? I have already erased and re-loaded it and just started having the problem again. wah!