Sorry, but it's almost certainly your fault. The site uses a <!DOCTYPE> statement that sends browsers into "Quirks Mode". For optimum cross-browser compatibility, you need to use a complete <!DOCTYPE> statement so that browsers will render the page in "Standards Compliance Mode" so that they're all at least trying to render the page according to the W3C Stanards. Search on 'doctype switch' and you'll find some good advice.
You may also be running into differences in the default margin settings for some tags being different across browsers, which can have a major impact as well. Good luck!