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
Mike