Java, GreaseMonkey scripts (read about a case not too long ago, maybe at mozillazine, a script of few lines ate up near 100 Mb), damaged or corrupted profile, driver (AFAIK, Firefox and the nVidia drivers aren't on friendly terms), cache overload (might slow browsing), and so on.
And of course FF is not perfect, so one of your more visited sites possibly has a code part what triggers the bug and results in crash.
I was being facetious, but thanks. =)
Point being that FF was crashing and I moved to another browser. And unfortunately when you have this crazy rapid release schedule, the plugin and driver developers cannot keep up, so you have to wait to use stuff, or disable it, or not go to certain sites. Eventually it just got annoying.