Not specifically on Arch, same problem in Windows.
At first I thought it was the finger print protection, because that has caused it before (as far back as 2018 according to Mozilla bug tracker).
BUT no, not this time.
Does it work normally other than that? And just to be clear, you are talking about having the FF window taking up the full screen (with tool bars, address bar, etc showing), not the window being in “full screen” mode, which I think would be something used to watch videos, etc.
To clarify…Just realized…
I have been using the Firefox Nightly version and the behavior is consistent…Not maximized…
I just opened up the Firefox Stable version and it works normally…
No idea…But for me…Survivable
Peace and health