This issue has something to do with the animation. When I replace the animation with a simple show() method, there's no pixel shift anymore..
* There's some strange submenu text pixel-shift (FF only) and right border redrawing (both IE and FF) going on, which I can't explain. However, this also seems to happen using the original Superfish menu! Maybe this is a common jQuery animation related bug? I have already tried the beta of 1.2.2, but unfortunately that didn't help..