Firefox HTML5 Fullscreen without the screen going black

[Blackbeat]

I'm using Firefox 16.0.1 and watching a video using a HTML5 video player.
It has a fullscreen function, too.
But other than with Flashplayer, my system doesn't recognize that it's a video playing and therefore starts my screensaver/blacks out my screen.
It's quite annoying to change the windows energy mode every time I watch a video, in order to hold it from turn on energy saving.

I'm curious if that's caused by sandboxie blocking a communication path of Firefox or HTML5 itself.
Would be great to find a way to solve this.

Regards

[Blackbeat]

Okay, I found the reason.
For everyone else who is interested about that topic:
This is a missing feature for Firefox as documented here.
I hope it is being added in the near future.