Some of our users, myself included have awoke one day to find the site missing the all important scrollbar, the ability to scroll with the mouse wheel and completely non existent silverlight on Firefox.
I have determined the culprit to be the addon: NoScript.
More specifically it is the allowing of 'cloudflare.com' that appears to be the cause of this. If you examine some of the hijacked scripts on the site with the debugger you can see cloudflares monkeying has severely buggered a lot of things, I don't enjoy their insertion of their own widgets myself, that dictionary one haunting the shoutbox is irritating.
To fix this simply forbid 'cloudflare.com' and the site should automatically refresh, if not refresh it yourself and expect to be greeted by your old pal, the scrollbar.