Firefox has become very unstable over the years, particularly with the unfortunately ubiquitously-necessary Java and Macromedia plugins. I've had lots of browser crashes, a few BSODs and often a lot of lag and unresponsiveness.
Webmasters gripe about ad blockers, but most flash ads are super-bloated and buggy to the point of crashing Firefox a lot, and its even creeped in a bit to the same plugins on Chrome sadly, though nowhere near as bad as Firefox (and thus I tend to default to using Chrome).