I upgraded to High Sierra (Version 10.13) Upon clicking the full-screen button it filled most of the screen but left a small strip, the height of the dock, still showing the desktop. What you are seeing in the screenshot is SNL clip being played in 'full screen'. As in, not Firefox being put into full screen mode, I mean playing a clip from a streaming service & clicking the little button that makes the clip expand to fill the entire monitor. As you can see, in the screen shot, there is still a strip of the desktop still visible. There shouldn't be. When you click full screen on any video platform, it should expand to fill the entire monitor.