When I watch fullscreen videos in another browser (Chrome, Safari, etc.) on my Mac the video fills the entire screen. For some reason when doing the same in firefox the video does not reach to fill the entire screen and instead only occupies the middle. I tried screenshotting to illustrate this behavior but the screenshot was restricted to this middle zone as well. It seems like somehow firefox is shrinking the display size when no other browser exhibits the same behavior.

It also seems like if I want I can drag the video window around in fullscreen moving the whole video player around my display. I shouldn't have to drag the thing around to get the video to play right.