What happens when you exit full screen? Could you describe the problem in a little more detail, or upload a screen shot showing the problem?

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redtone said

After new update of Firefox for Mac problem occurs once you exit out of a fullscreen video on youtube and firefox exits out of fullscreen mode.

Okay, I get it now...

You are in Firefox's full screen mode. Then you put the video in full screen mode. Exiting full screen mode for the video also exits full screen mode for Firefox.

When I try it on Windows 7, I get the same thing, using the default "HTML5 Player" on YouTube. However, if I force the use of the Flash plugin with an extension (YouTube Flash Video Player), then exiting full screen in the player does not exit Firefox full screen.

I searched in the bug tracking system, and it appears this will be fixed in Firefox 40. According to a comment, the fix was too complicated to easily add to Firefox 38 or 39, so unfortunately, until Firefox 40, if this is really annoying, you will need to use Flash for videos (which you can force on YouTube using the extension mentioned earlier).

• Bug 947854 – Exiting DOM fullscreen also exits window fullscreen mode
• Bugzilla Etiquette

Thanks, I appreciate the solution you found for me.