Firefox problem

I've just had to reinstall XP (including SP2) and re-install Firefox (v 2.0.0.11) and since putting it back in every so often when I'm using Firefox the system just reboots whilst I'm using it! I can't figure out why. Anyone got any ideas. It isn't a hardware problem as it doesn't happen in any other programs...
The Mozilla forum seems to be obsessed with hardware problems causing it but as an ex-hardware engineer I know that it isn't that. I left the machine up all night checking the HDs etc and nowt is wrong with them...
The Mozilla forum seems to be obsessed with hardware problems causing it but as an ex-hardware engineer I know that it isn't that. I left the machine up all night checking the HDs etc and nowt is wrong with them...