Need help on the XHTML CSS box model

http://www.piercecollege.edu/community/aec/fall06_2.html

The page subheading are h2 tags with width and height and background color specified in CSS. The page displays as desired in IE but not in Firefox. I validated the html and css at W3C, so I am not sure why Firefox cannot handle a simple box model. What is it that Firefox wants that I do not understand?

If anybody can give me a clue please help, Thanks.