I've downloaded Firefox 4.0.1 recently, and everything works great except for Flash. I have Flash v10.2.159.1. I would be playing a video (doesn't matter what site), and two or three minutes into the video, it would hang the entire system, forcing a reboot, no error messages. I have reinstalled the Flash player for Firefox, uninstalled and reinstalled Firefox itself, cleared all the preferences, and disabled hardware acceleration... nothing works. Flash plays fine in IE and Windows Media Player, so the video drivers are not the problem.