Things that I have seen that can cause FireFox to cough...
1. The Configuration File--if you have not done much yet (lots of bookmarks, plug-ins, etc.), uninstall FF and wipe out the directories. You can save/move the bookmark file and put it back in the new install.
2. I have found that the NoScript plug-in can cause hangs/crashes on some sites -- especially those with embedded video and lots of scripts (that I have not white-listed). Mostly, just a couple of sites where I see crashes (and not often overall).
3. Check your URL History--Somewhere, my history got set to a very large number (months/forever?)... If I deleted a URL history link--it would sometimes hang then crash.
4. I have found one add-on (extension) that makes my FireFox very buggy. "Cookie Safe v3.x.x"... Don't know why, but my computer (windows XP) really slowed down (and locked up machine sometimes--as I recall) when that little add-on was enabled--perhaps, one of your's is causing grief (disable 1/2 -- run, then re-enable and disable the other 1/2, test run again).
-Bill