It just started showing up a few days ago and although it's not on every site the ones where it shows up it's very annoying. It started after I upgraded to 3.6.15 so I did a system restore thinking it was something with the computer and went back to the browser version before that and it still had the new horizontal scrollbar. When I opened Firefox this morning it automatically upgraded back to 3.6.15 and it still does it, most notably on Facebook and Yahoo but on other sites too.