I "upgraded" to Firefox 2.0 back in early December and have regretted it ever since.
After some finite amount of webbrowsing, Firefox simply stops working. It will no longer connect to any websites at all. I then ctrl-alt-delete to close all of my open Firefox windows and restart it. Thank GOD it has the "restore sessions" option so, at least, it re-opens the pages I was last looking at, but it's really ridiculous and has pushed me back to IE.
I did some searching in Dec and found that I'm not the only one with this problem. There is a whole list of things that might cause it, 99% of which do not apply to me. I'm not using any plug-ins or anything else. Just vanilla Firefox. I went through a process of uninstalling it completely and removing all registry info related to FireFox (as some of the fix-it websites recommend) so I could install from scratch. No dice. It works for about 30 minutes of browsing and then I have to shut it all down.
Right now I'd have to say that Firefox 2.0 (2.0.0.1 to be exact) sucks big donkey balls. I wish I'd never 'upgraded' from 1.whatever.