You can toggle full screen mode by pressing F11 or Fn + F11 (Mac: Command+Shift+F).

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

If you are in full screen mode then hover the mouse to the top of the screen to make the Navigation Toolbar and Tab bar appear.
Click the Maximize button (top right corner of the Navigation Toolbar) to leave full screen mode or right-click empty space on a toolbar and choose "Exit Full Screen Mode" or press the F11 key.

You can also check for problems caused by a corrupted localstore.rdf file.

• http://kb.mozillazine.org/Corrupt_localstore.rdf

I hit the drop down tab on firefox and hit FULL SCREEN. I don't like it...how do I undo it. I hit Fn, Fn + F11

I am using windows 7

Do the the Navigation Toolbar and Tab bar appear if you hover the mouse to the top of the screen?

Click the Maximize button (top right corner of the Navigation Toolbar) to leave full screen mode or right-click empty space on a toolbar and choose "Exit Full Screen Mode" or press the F11 key.