That's strange, usually if you scroll down to where you can see the title below the full screen video player, the YouTube toolbar shows at the top. I'm not sure what is happening in your screenshot. If you scroll up/down/up can you get back to regular full screen?

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?