I do have FF to crash on me every once in a while. Each time it is something about the site, though. It is usually a video that crashes it, but not something like YouTube... something funky that is using its own player or something. I just restore the session and kill the tab that caused it (I use a lot of tabs).
Yeah, Firefox gets tired every now and then and just needs a restart. I think that it has some memory leaks or something.
For me there are usually warning signs; a page won't load quite right, or a function or two on the page won't work. But that's usually after a couple of hours of surfing or a bad page.
You know how some picture pages/artists LOVE to put several hundred images/flash objects on a single page? Those are the people that I just want to strangle to death with their own keyboard cord.