The problem isn't Sepia - I'm getting the same margins on mafSilver in FF12 on Snow Leopard. It might be Firefox, though - when I open Camino 2.1.2 or Safari 5.1.7 on the same machine, I don't get the margins.
Discretion is the better part of valor. If I helped lynch you, you deserved it.
In post 8, Mr. Flay wrote:Odd - FF 12.0 on Linux doesn't do it.
Took me a moment to realize this, but I'd look to make sure you don't have a scroll bar at the bottom. The content is still the proper width it just has the extra margin space. Don't know whats generating it yet.