I have that problem with FF at work, but not at home. I guessed it had something to do with the fact I have to set my cache here at work very, very small (500 KB) because it saves in my profile instead of locally and I run out of profile space right away.
I use adblock at work though, so after a few threads freeze on me (it usually responds eventually, but it takes like 5 minutes) I adblock all avatars. It's almost always one avatar that messes FF up for me, but once I block it the threads are fine... until I hit another bad one. Because I'm at work I don't want to see avatars anyway, so it works well for me.
But that never happens at home, and cache size is one of the only things I can think of different at all.