Mmm nope sorry mate, thats probably a problem a the site end and you just happened to catch it in firefox.. since I never use IE, and I've never seen that error.. despite hitting the site more than most
It looks like a PHP error, we'll have a look -- but its not firefox thats for certain.
It is server side generated error, still it has occured to me that it is generated when behind proxies with specific configuration and only in (all) php-nuke scripts! I haven't looked at it any further.
So it would make sense if you have forgotten some proxy at your firefox configuration.