Crossbrowser position:absolute

Hi at all,
I have a little problem about position:absolute.
I want to position absolutely a menu bar.
if I write:
menubar {position: absolute;top:5.2em;}
with MSIE the menu bar is too height on the screen and is right on FireFox
elseif I write
menubar {position: absolute;top:100px;}
the menu bar is right with NSIE and not with FF
How can I do to have a crossbrowser result of my menu var?
Thank in advance

Pablito