> It's Firefox. So I guess it's either a fault of the browser then.
Maybe. I think it's the issue with what viewport size the browser decides to use. The game window is standard 16:9 size, however I've to make the iframe taller so that the fullscreen button at the bottom is visible. It seems that FF uses that iframe size when it scales up the viewport, while Chrome uses the webgl's viewport size. I'll see if I can fix it somehow.
Either you can try using Chrome or sign up for playtesting on Steam (I just set it up, you will soon have access to windows beta builds through it)