I wonder whether this a problem with the F11 key being blocked/intercepted by the site or an add-on. Could you test whether the full screen button on the menu works -- it is the last one on the Zoom row.

I don't think F11 or Firefox's built-in full screen is relevant, so please ignore that. The display in your screenshot is somewhat similar to when you scroll down, but presumably you didn't scroll down. Does scrolling up/down/up make any difference?

There are a couple new threads on r/Firefox about this, too:

• https://old.reddit.com/r/firefox/comments/1gvdjuv/youtube_full_screen_issue/
• https://old.reddit.com/r/firefox/comments/1gvexbq/youtube_not_actually_going_fullscreen_when_trying/

Users there reported that if they are signed out or using a private window (where, presumably, they didn't sign in to their accounts), then the problem doesn't occur.

Maybe Google made a change?