I was in a presentation a while back given by the head of Microsofts new security team that has been in place for a few years now to make sure security concerns are addressed across all products they make.
What he was trying to stress about non-MS browsers is that they don't get hammered like MS does by hackers. Every MS product is constantly attacked by hackers requiring MS to be way ahead of the other companies out there on security and trust me when I say they are. My last job was writing network security software that is used by Missel Command, DEA, MS, etc... so I am speaking from an area of professional expertise.
Anyway, Firefox has been concentrating on features as that is the only thing that consumers see and they realize that once you win market share you can then address stability and security. The crashing issue that Vegan pointed out could very well be related to this lack of thorough testing on Firefox's part and the much lower incident of hacking that they experience.
Don't get me wrong, firefox definitely has great features (which MS will bite) and I like it a lot for web developement but I use IE for all my browsing just because I know it's the most secure browser on the market.