The crashes, apparently, are caused by Firefox allocating more and more memory for web pages, DHTML objects and such, never freeing any and, once memory runs out, dying horribly. Apparently our technological civilisation, which has put men on the moon and mapped the human genome, is incapable of implementing a web browser that does not leak memory like a sieve and spontaneously die from time to time.
I've heard it claimed that Mozilla/Firefox's memory leak is not a bug but a feature; the theory being that it's Nature's own caching mechanism, ensuring that the browser runs more and more efficiently (at least until it exhausts all system memory and dies, that is). Which is a nice piece of sophistry.
Anyway, for those using Firefox, there is a minor salvation in the Tab Mix Plus extension's session management facility, which saves your session and, should your browser crash, offers to restore it for you. Of course, should Firefox happen to die when reloading all the saved pages, it could be a problem, but not to worry: another feature (and definitely not a bug) is that, if that happens, Tab Mix Plus throws out some of the pages (seemingly at random) before the next attempt; thus, you eventually get to a set of pages which will reload without crashing, and all is well with the world. What would we do without such self-regulating mechanisms?