Check out the solution in this thread: How do i exit fullscreen mode on a tablet PC operating Win8.1 and no physical keys?

In short:

• Open the charms bar by swiping in from the right side
• Tap Settings, then at the lower right, the icon to call up a virtual keyboard
• If there is not an Fn key in the lower left corner of the virtual keyboard, use the keyboard selection button (lower right?) to try other available keyboards until you find one with Fn*
• Tap Fn to change the number row to function keys and then tap F11

Does that work for you?

* If the standard/full keyboard is not among the options, another article says you can add it as follows: Change PC Settings => PC and devices => Typing => Touch Keyboard => Add the standard keyboard layout as a touch keyboard option => Turn on

No idea if it work with Windows 8 or touch but from early Windows probably Windows 3 and certainly XP to 7 keyboard shortcut Alt + Space may work to give you control options.

• It's still listed http://windows.microsoft.com/en-gb/windows/keyboard-shortcuts#keyboard-shortcuts=windows-8

I just had this problem with the latest FF on my oldish Windows 7 Dell desktop. I had a lot of tabs open and at one sometimes troublesome site, SFGate.com, went to fullscreen to do a screengrab, then couldn't get out of it. Escape nonfunctional, no clicking or hovering at the top worked, no help in the rightbutton menu, nothing to get back to ordinary FF screen. I didn't know about the F11 fullscreen toggle--but of course couldn't get to Mozilla support with FF fullscreen locked. I rebooted FF several times (via Task Manager), finally rebooted Windows. Same problem with FF: as soon as it came up, it was locked in full screen. Killed FF, went to IE to download FF, reinstalled it. That worked, though of course I lost all my open tabs.

If it happens again, I'll try the F11 key.

Was it a problem of too many tabs, too much FF in RAM, or maybe weird SFGate.com?