Firefox wont open Videos in "full" screen

Whenever i try to open YT Videos or other in-browser videos in Full-Screen, the video only fills the screen to like 95%. The top and the left have a small bar, which just shows my desktop background. (See att.) Closing the Full screen window and resizing the firefox window, repairs the issue. But after a restart it happens again. Enabling hardware acc. kinda fixes it, but not consistently. And i would prefer to not use hardware acc. A reinstall did not help and i dont seem to be the only one with this issue.