How to Exit / Escape Full Screen Firefox?

How can I escape / exit full screen in Firefox?
My cat accidentally jumped on my keyboard and at that point Firefox browser was open. I have no idea which keys it stepped on, but my Firefox changed mode to full screen.
However, I managed to come out of the full screen mode of Firefox.
To exit full screen mode of Firefox, please press the “F11″ (Function 11) key.

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