Yeah, the definition is pretty poor on that screenshot, don't know why! Anyway, can just about see what's going on, and that you're using Firefox.
It could be to do with a Firefox extension that you've got installed, as I've just tried the forum in Firefox and it works fine (i.e. no horizontal scrollbar).
You could try disabling your extensions (Tools / Add-ons) one by one until you find the one that's causing the issue, if that's what it is.
Good luck