Pressing the F11 should do it.

In most cases F11 will full screen it. However, I have noticed on screens with certain scripts (i.e. java) the F11 key will not respond. If this happens just open a new tap to a blank page or something like google.com and press F11 on that screen and you should switch to full screen mode.

Also if you go to the top right corner of your screen, and right click, there should be an option that lets you exit full screen.

You can also hover the mouse to the top of the window to make the Navigation Toolbar visible and click the Maximize button to leave full screen mode.

• https://support.mozilla.org/kb/how-to-use-full-screen

Okay, I had this problem too. Read Their answers none helped.. FINALLY i pressed random buttons.. So what you gotta do is go to the RED X in the top Right corner RIGHT click and the last thing it says should be "exit full screen mode". How you get yourself into full-screen anyways, is f11 i believe ;3 Hope this helped..