Very Minor 6 Pro Firefox bug

I got my 6 Pro yesterday, and I've only run into 1 problem so far.

If I open Youtube in Firefox, then go to fullscreen mode, the video is not displayed correctly. There is a small black bar on the top of the video, and the bottom is cut off by the edge of the screen.

I have confirmed that the Youtube app is able to display fullscreen videos correctly, with no unnecessary border.