I believe that that is an issue with Flash, not with xmonad. Xmonad only lets applications draw to as much of the screen as then are alloted. If you fullscreen

I believe that that is an issue with Flash, not with xmonad. Xmonad only lets applications draw to as much of the screen as then are alloted. If you fullscreen Firefox, it will simply hide UI elements, not take up the entire screen. It is not xmonads responsibility to automatically scale applications.