I don't see the issues on the stylesheet either and my site also displays slightly differently in Firefox but to me it isn't an issue and Firefox tells me it is being displayed in standard compliance mode as is your site.
What is a bigger issue is the fact when trying to login using Internet Explorer a message pops up to say some items on the page are insecure. Now i used to get that and fixed it by making some of the image links https instead of http.
Might be worth investigating the latter problem more as that sort of message is off putting for customers.