Are you installing any add ons or extensions such as firefox uses? If so, uninstall firefox using a program like revo uninstaller. Maybe also uninstall java, flash, etc. Reinstall firefox, verify that it works, then add in things one at a time and see when the machine crashes. Also, do any other programs exhibit this behavior such as Internet Explorer or any other programs?
I don't agree. You can uninstall everything and of course if the problem is flash or java it's not going to crash but the reason it's not crashing is because it's not using them. But as soon as you reinstall them you have the same prob. though these days you can't run the internet without them. So that's not a good solution.