If you look at the source view of that site, xarart, you see this at the bottom... it is not valid so it throws an error for Firefox...
IE don't care what you throw at it it just does what it thinks you want... some times good some times not...

line 53 column 1 - Warning: content occurs after end of body
What's with all the extra crap, how is that any way to treat a customers web page?...
I've used 1and1 for years and customers also use it with no problems...