IE is garbage generally.
You see everything before IE8 is not standards compliant, IE does it's own thing.
Firefox, Safari, Chrome and Opera are all standards compliant.
Firefox looks at your code expecting you to know what you are doing. It shows the site as you wrote it. If it is not how you want it, then it is because you wrote it wrong.
IE assumes you do not know what you are doing and it shows you what it thinks you want and ignores your mistakes.
So you have done something wrong in as far as where things are and Fx is showing you what you wrote, showing you the mistake. IE is hiding the mistake from you and showing you what it assumes you want.