This is expected behavior. However if you would like to see this change I would be happy to file a bug with the expected behavior attached. Can you please provide the steps you take in order to do this?

I also ran across another work around today: It looks like there was an about:config entry that was created, but is no longer in the current build. It may be possible to create a new entry and add it back: http://www.shdon.com/blog/2012/11/24/browser-tip-keep-video-full-screen-w...

Made via: bug 724554

Please be warned that this will change the default of your installation, but I do not know how this will affect other functionalities of the browser.

Sry, tried but can't find the full-screen-api.exit-on-deactivate command.

aluca said

Sry, tried but can't find the full-screen-api.exit-on-deactivate command.

turned on and off all full screen commands, back to normal : )