Positioning Elements between browers.... *sigh*

One thing that really bothers me about working with IE and Firefox is, while using Top/Left to position objects on the page (in CSS), it will appear correctly in one, but not the other. I try switching between 'px' and 'em' and other units, but really have no success with this being consistent from one browser to the next. Any tips on lowering the standard deviation on this process? I'm trying to position a row of tabs on the screen. When I put a margin of 10px on the far-right tab to give it a margin from the edge of the screen, the margin is much bigger in IE than in firefox. I can't seem to fix this.

Any pointers?

dep

Why even use positioning in the first place? Almost always there is a better way. Post your code or a link to the site, the site link would be better.

well, it's a very complex page layout with several layers working independently of one another. i tried to minimize the use of top/left, but still have to sometimes.