Yesterday I shutdown my PC running Ubuntu 18.04 while Firefox had a full screen window on each of my two monitors. When i turned it on today the window on my right hand monitor opened correctly. However, the window on my left hand monitor had these problems (see the uploaded image) it was not full screen. It was split into a window at the bottom of the screen that showed the expected contents and a black window at the top of the screen that took mouse input. Note the 'KCOM webmail...' popup by the mouse cursor over the upper window. A right click on the desktop had no effect on the left hand monitor, as if something considered the window full screen. After restoring down the window on the right hand monitor a right click on that desktop showed the expected menu. I recovered from the problem by moving the mouse cursor over the black window until the 'Maximise' popup appeared and clicking. The window then became full screen and has behaved correctly since. I have xpdyinfo and xwininfo outputs in case they are useful. I upgraded from Ubuntu 16.04 to 18.04 about 2 weeks ago and have been using Firefox almost daily since then. I have often seen it restart correctly when I restart my PC, although perhaps not with two full screen windows. I don't know if this is a known bug. I hope this information is useful.