I use Firefox on Rocky Linux (formerly Centos) and the forum comes out very narrow on my screen, using only about 1/3 of the available space in the Firefox window, as you can see in the attached screenshot.
I just tried it with Chrome on Rocky Linux and the forum looks normal there; it occpies the entire browser window as it should.
The forum is usable on Firefox, it's just really skinny!
I'm utilizing the myBB forum software - it's possible they don't properly handle something on Linux with Firefox. I tested with Firefox on Windows and it seems to work fine for me. I don't really have any answers unfortunately... I can try to raise an issue with the myBB developers to see if they are aware of any issues. You can also try adjusting the theme with the dropdown at the bottom right corner to see if the default theme, or one of the other themes, works correctly.
Mike
Well, that's really interesting.
I tried your suggestion about changing the theme, and the first change (to the first choice past default) changed nothing; the second change (1point8) widened the screen out to what it should be (yay!) and after that any change (including back to default) keeps the screen at full width.
Weird. But that fixed it!