I checked out the site in Internet Explorer in addition to in Mozilla Firebird. Just as you said, it loaded fine in Firebird but IE didn't like it very much. The HTML code doesn't look malicious at all, so I don't know why it wouldn't work.
The only thing I could think of is that server-side there is code to display different things depending on the useragent. (i.e. what browser/os is being used to load the page). This is plausable and I've seen it done before. I just don't see why it would be done in this case.
In addition, what gave you the idea this site was in php? I don't see any files ending with a *.php extension?
I'm pretty sure it's in PHP because I did see some pages with a PHP extension. I also saw pages using the PHP session IDs to track user sessions.