Because IE and FF render a bit differently, specially if there is no valid Doc-type defined for the page. Then IE will go into Quirks mode, and do its own thing on the page.
Research Doctypes, and Valid webpages, and IE's Box model for more information on the subject.
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Ignorance is not necessarily Bliss, case in point:
Unknown has caused an Unknown Error on Unknown and must be shutdown to prevent damage to Unknown.