That's a truly excellent tip, I definitely need to go tweak that setting.
As an additional tip, if you want to get rid of the "You are now fullscreen" (or whatever it says) that comes across the top... you can set the following to 0
full-screen-api.transition-duration.enter
full-screen-api.transition-duration.leave
full-screen-api.transition-duration.timeout
full-screen-api.warning.delay
full-screen-api.warning.timeout
What a wonderful but bizarrely named feature flag. Thank you!
then, when you press `F` on a video, you will remove all firefox 'decoration', just like fullscreen mode, but it'll respect whatever position and size you have set for the browser.