Thi is a a well-known fact.Every one know that Firefox and IE have same difference.Especially in DIV tags Firefox and IE have different absolutely value.
But with one CSS u make combine.
Not undestant u request very well.So u tall that see same different in look on IE and Firefox.

I have see u style.css and see many excess in him.
If u want to make one stylesheet just put
html* before class or ID for IE.
And give different value for this ID or class.
For exaple
html*div.exaple{
margin: 10px 10px 0px 0px;
padding: 0px 0px 0px 0px;
}
div.exaple{
margin: 5px 10px 0px 0px;
padding: 0px 0px 0px 0px;
}
First is for IE second is for every one browser.