You should provide the link so that we try to replicate the issue. I don't see that issue with YT in full screen mode. see screenshot

Not sure the link would work for you because when I copy-pasted the link in other browsers, it goes back to the standard YouTube setup. But here you go anyway.

https://www.youtube.com/watch?v=MBo4GViDxzc

See first screenshot for my full screen mode. It doesn't change anything for me.

Also noticed that the progress bar for the Miniplayer mode gets placed at the top section of the Miniplayer, See second screenshot. And just for reference, the third screenshot is showing my firefox version. It seems the same as yours.

I cannot replicate the issue. Did you try Firefox in Safe Mode? https://support.mozilla.org/en-US/kb/diagnose-firefox-issues-using-troubleshoot-mode#w_how-to-start-firefox-in-4troubleshoot-modesf5safe-modesf

Where did you get Firefox from? What security software are you running? Can you try under a different windows account?

Edition Windows 11 Pro Insider Preview Version 22H2 Installed on ‎20-‎Jul-‎22 OS build 25163.1010 Experience Windows Feature Experience Pack 1000.25163.1010.0