first of all, from security reasons it´s a really bad idea to place an ftp-server on a firewall. FTP-servers can be easily exploited to gain acces to the system. And then your firewall is hacked!!
If you really want to set up an ftp-server, use a dedicated machine. And i think i remember, that a description of what to do is part of the man-page of ftp (command: man ftp [:)] )