You can do one of the following:

• Tap the F11 key to turn Full Screen Mode on or off.
• Move your cursor to the top edge of the monitor, tabs and some toolbars wil show, right-click in the empty space at the end of the Tabs, click "Exit Full Screen Mode"
• See: http://kb.mozillazine.org/Netbooks#Full_screen

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I must have hit the F11 key in error while swapping keyboards. What a shock !

I have been using Chrome for quite a while for most things and had no idea Firefox had this support available.

I had looked for it & came up empty.

HOORAY ! I love it . . . .

Please never take it away ;)