In FF pressing space to accept the "start" or "restart" buttons, the window gets scrolled down. pretty annoying on the 6th level since i was already dead a couple of times till i was able to scroll up again. Next thing i recognized was, that the player walks kind of slow while running against a wall sideways. Like there were a lot of friction. Since a already can't pass level 6, i can't say something about the difficulty of the other levels.