Fresh Mondrian template – CSS problem

I’m using the Fresh Mondrian template from Alex King’s site. In IE, when the page of posts is longer than the menu, the menu ends, leaving the background colour visible. In Firefox, the menu is always the same length as the page of posts. The problem occurs when the page of posts is shorter than the menu. In IE, the menu will continue on its own background colour whilst in Firefox, the background becomes transparent and the menu options illegible. Been playing around for hours trying to find a solution by trial and error, but no success. Can anyone help ?