I haven't looked at your code closely, but...
Absolutely positioning between IE and Mozilla (and Safari) are a couple pixels off. This is because, as I understand it, one browser sets the absolute by the HTML tag, the other by the BODY tag. While this shouldn't make a difference, it does. There are also differences if you start using borders, padding, margins, and so on - each browser handles the additional space that each element takes up a bit differently.
You can try setting your HTML and BODY tags to have borders of 0. If that doesn't work, then you may want to change how you're do