Most firewalls allow you to create a "virtual server" or map a static ip from external to internal. You can get into your firewall and check if you know the user and password. If you know the ip address of the firewall enter it into the browser. If you don't know it's address you can find out what it is easily enough. For win98 start->run->winipcfg, it's the default gateway. For Winxp start->run->cmd, from the command prompt type ipconfig. Again, it's the default gateway.
By default, many firwalls use admin for the user and for the password use the name of the manufacturer or blank or admin. You likely received documentation with it that will tell you the default login.
Also, keep in mind that by creating a virtual server you are effectively eliminating the firewall.