I just upgraded to Firefox 4.0 on Friday - 2 days ago. My machine has intel processors, running Windows XP. After almost every mouse click, the browser hangs, and the title bar says "Window title (not responding)," where "Window title" is the actual window title. After waiting a minute or so, the windows repaint and clicks are again accepted. But, again, another click, or sliding a scrollbar will cause the same hang (and same wait) to happen again. Typing text seems to work OK, though. Youtube videos also don't work. The controls below the videos are missing.

Processor use and network traffic are both at or near zero during the hangs.