I probably localized the source of the problem of annoying top-left gap in fullscreen mode videos. In my case (I use 2 monitors) that gap shows up when in Settings -> General -> Browsing the setting "Enable picture-in-picture video controls is unchecked. When I have it checked the top-left gap doesn't show up when turning video fullscreen mode. Also the gap doesn't show up when I have only 1 monitor connected. So it may be a problem with multiscreen display and Firefox's picture-in-picture. I hope my "solution" will help anybody. I'm writing about that problem: https://www.reddit.com/r/FirefoxCSS/comments/111bv0d/fullscreen_gap_bug_can_somebody_fix_this_firefo...
