I have been experiencing regular screen freeze-ups every time I attempt to install a new plug-in or add-on to Firefox with Compiz-Fusion installed and running. This happens with Firefox 3.0.10 or with Firefox 3.5 (Shiretoko) The cpu goes to 100%, and the entire screen becomes unresponsive, including all taskbars, window closer buttons, everything. The mouse cursor will move, but you can’t do anything with it! This means rebooting with the power button every time.
As a workaround, I installed “Compiz Fusion Icon,” which toggles between Metacity and Compiz window managers. Unfortunately, you have to remember to use it BEFORE you try to install a Firefox extension! I wonder if this is related to npviewer.bin. Just a wild guess, since there’s no way to open System Monitor while the screen is frozen up!