Div-Layers are on differen positions in IE/Firefox

Hey Guys,
I think this is a very known bug or whatever it is, but I don't know
how to work around it;
The round-edges on this website - www.bildungswerk.oekolea.de - are in
two layers (generated in Dreamweaver), and are shown properly in IE,
but not in Firefox.

<div id="Layer2" style="position:absolute; left:10px; top:10px;
width:28px; height:29px; z-index:2"><img src="img/obenl.gif" width="28"
height="29"></div>

<div id="Layer2" style="position:absolute; left:730px; top:10px;
width:30px; height:29px; z-index:2"><img src="img/obenr.gif" width="30"
height="29"></div>

That's the code.

What do I have to do, to have em displayed in the same position in both
browsers?

Flo.

Oh come on it can't be that hard ;-).