Doesn't seem to be an add-on, like I said it'll happen at random times, even when I'm not at the computer or when I'm using another program. I have 4 GB of RAM but thanks to 32-bit XP and background programs I only have about 2.3 available when I start my computer, but I never let my available RAM dip to dangerously low levels. Luckily it hasn't crashed again since I made that post, likely due to the fact that I'm now trying to keep the number of tabs open under 50, but I'm still considering switching back if it starts crashing again.
Any 32-bit program on XP is limited to a max of 2 GB memory anyway, so running out of memory is doubtfully a problem. If I were to guess, I'd put my money on adobe flash and Firefox's new sandboxing causing the crashes. Flash is [censored].
Still, though, I don't see how you can maintain any level of control on 50 tabs. You can't even see them all at once much less actually use them all at the same time in any active manner. The worst my browsing ever gets is when on an image board. I'll have maybe 30 tabs open. Then I go, save all the pictures in the open tabs, close them, and revert to my usual 4 open tabs.