Steps to reproduce: 1. Open two windows, one in the main monitor, the other in a secondary monitor 2. Open https://developer.mozilla.org/samples/domref/fullscreen.html in the window in the main monitor 3. Click fullscreen button 4. Click the other browser window to activate it There are several stage of expected result: 1. (best) the main monitor still shows neither menubar nor dock, and the secondary monitor shows both of them 2. (probably reasonable) the menubar and dock shows on both monitors. note that we always get this behavior if user activate another application instead of the other window of the browser. also, this is the current behavior of Chrome. Actual result: The menubar and dock do not show in any of the monitors when any Firefox window is activated. I'm not quite sure what can we do for this. I guess it is almost out of our control. But given Flash seems to do something better, we can probably also improve ours.

This is fairly annoying, and actually has existed for ~7 years: https://bugzilla.mozilla.org/show_bug.cgi?id=635089

Oh, so this is a duplicate of that bug.