So I'm on the new Macbook Pro (2021) with the newer screens. Full screen does not take up the whole side of the displays. It looks like the older Macbooks with the wider bezels when its in fullscreen on any video in youtube, hulu, disney+, netflix ect anything that goes in fullscreen with a video playing.
Edit: So if Firefox is in full screen (green bubble Mac OS), videos take up the whole screen. If Firefox is in a window and you hit full screen on a video then its cut off on the sides. Kind of like if the bezels were fatter on the laptop screen.
@JfishinI understand your point, I have the same problem... Very annoying that no one has solved or perceived this problem.
Search about:config in the url bar. Then search full-screen-api.macos-native-full-screen
set that to true. I should have posted the fix when I found it but didn’t think others had the same issue lol.