I'm using a new Mac Powerbook with 10.6, latest updates. I'm also using a secondary screen hooked up via a Plugable brand dock. Clamshell mode doesn't work (Apple problem) , so I have to leave the MacBook open, but screen mirroring generally works well, with black bars at the top and bottom of screen because of different native resolutions. The problem happens when Firefox (or any browser but Safari) is launched. The screen goes blue and a secondary "ghost" menu bar appears at the top of the screen and remnant graphics appear on the bottom bar of the screen. Both of these areas remain inaccessible to the mouse. The appearance is like a second, light blue, menubar is at the top of the screen. The desktop image is also blanked to solid blue. It doesn't impede functionality, but to return the desktop to normal, I have to quit the browser, go to System Preferences and reset the screen resolution to something else, then reset it to the original resolution. The recalculation seems to "remind" the computer about desktop settings. Tried zapping PRAM to no avail.