Recently my small dog ran over the laptop keyboard. Since then I have been unable to get Mozilla to exit full screen mode.

F11, no matter how quickly I press and release it, acts like it is trying to get out of full screen mode but returns to it in a fraction of a second.

Fn+F11 does nothing.

I've clicked on "exit full screen" by right-clicking on the top right menu icon, with no results.

I went to prefs.js and changed everything, one by one, that came up on a "full screen" search, exited Mozilla and started it up again with the changed values to no avail.

Everything, including Mozilla and the OS, are current.

Any help would be appreciated.

Take a look at your plugins. Try disabling them one at a time and testing Mozilla after each disable action until you find the offending plugin. One reference I found suggests it's the kiosk plugin.

If we assume that screen size is correct and that ff is running within the screen size then maybe remove and re-install if you can't get easy fix.