I've got no idea why Firefox is crashing. Some web forums suggest that it's vulnerable to certain types of spyware. (Run Adaware and/or Spybot Search & Destroy to eliminate this possibility).
However, I do know that most Firefox problems can be solved by reinstalling the program. There's no need to uninstall first and you'll find that all of your bookmarks will be preserved.
Chris
(PS: Just a thought - Have you started visiting any new sites lately? There are a few badly coded sites that seem to cause Firefox to crash. Picasaweb is probably top of the list)