On Netscape, the url http:/cgi-bin/wwwgnats.pl (notice one slash after the colon) loads the URL http://current-site/cgi-bin/wwwgnats.pl. On Mozilla, it tries to load the url http://cgi-bin/wwwgnats.pl, which of course fails. It'd be nice if it went with the Netscape behaviour.
It's a dup of 32966 which is marked WONTFIX. I suggest you read that bug page and make any comments you deem appropriate about the issue. *** This bug has been marked as a duplicate of 32966 ***