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As for those saying it doesn't work,
type "about:config" in the location bar,
at the message warning just hit "Enter",
then in the search bar type "full-screen-api.enabled" and change the value to true.
Now youtube fullscreen will work.

This is not necessary because this extension already sets full-screen-api.enabled on installation and on every browser startup. If someone has problems they must be caused by something else.