Not working in Mozilla Firefox outside VPN


I am using Sharepoint smart 404 in my sharepoint site. When i am trying to access a invalid URL from mozilla firefox 3.5, the redirection works fine if i am inside my VPN (i.e. intranet) but mozilla is showing the html code of smart404.html as output while accessing invalid URL from outside of VPN ( i.e. over internet). The same invalid URL is working fine incase of IE (both for internet & intranet).
The out put is as follows..
<head> <meta HTTP-EQUIV="Content-Type" content="text/html; charset=utf-8" /> <meta HTTP-EQUIV="Expires" content="0" /> <noscript> <meta http-equiv="refresh" content="0; url=/_layouts/spsredirect.aspx?noscript=1" /> </noscript> <script language="javascript" src="/_layouts/1033/init.js"></script> <script language="javascript" src="/_layouts/1033/core.js"></script> <script language="javascript"> var requestedUrl = escapeProperly(window.location.href); var url_array = window.location.href.split("/"); var lastelementindex = url_array.length -1 var pagename = url_array[lastelementindex]; STSNavigate("/utility/404.aspx?oldUrl=" + requestedUrl +"&k=" + pagename); </script> </head> <body>