In firefox for windows (and I think also in firefox for android), the Board index line is not placed correctly which makes it larger, Also when the browser window is resized, the top display of th forum just isn't long enough, or I will say the forum is still set to display on a width based on the screen window size it seems (basically, the forum d ecoration is set up to display inside the current viewport, but with those two issues, the forum actually looks bad).
I think the first also happens on firefox on android, which basically is annoying as sliding the forum makes it scroll horizontally too.