Forum Moderators: not2easy
To fix this, give your document a proper DOCTYPE declaration. Doing so will tell IE to behave more like Firefox (it puts IE into standards compliant mode).
Lastly, you wrote "fine in IE but not in FF". This is actually backwards. Take a look at the CSS spec on the box model and you'll see that IE is actually misbehaving (this is usually the case when there are cross browser issues):
[w3.org...]