On a site I am developing ( www.fysiotherapiedorst.nl ) the Template shifts a little bit (to right and left while browsing throug the pages) if I look in FF 3.5 but in IE8 it works perfect. Is this an CSS issue?

I don't see any difference. Is it possible that a scroll bar is appearing and disappearing depending on the page you are viewing? This can cause what looks like a shifting to happen