Because you used IE to create the site. Never, ever do that. IE is 12 years behind all other browsers in modern standards and compliance. Always, always use a modern browser. ANYTHING but IE. The multitude of quirks and bugs in IE are well known, as are the hacks to fix it.
In addition, you aren't using a doctype. This puts IE into 'quirks mode' and it won't attempt to perform like the other far more modern browsers, like Firefox, Chrome, Safari, Opera and just about everything else. Add this to your first line:
- <!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN"
-
"http://www.w3.org/TR/html4/strict.dtd">
Then let's see where we stand.
IE is your problem, not Firefox.