My PC does this. I use Internet Explorer (no Firefox) and I crash all the time when playing movies in IE. I've noticed that it only does it when I either get an IM, open an IM, or get a new IM, however. IE will not crash unless some type of instant messaging occurs while a video is going. The issue is certainly not Firefox-only.
It's been doing it ever since I formatted. I like how my crapped up installation worked fine with IE but now since I've formatted and got ALL of the updates it starts doing it.