Port redirection on server 2003

Router
I have a public IP: 1.1.1.1
Internal IP: 192.168.88.1

My firewall / NAT server
Internal / Internet: 192.168.88.2
office LAN: 192.168.55.1

In my office lan i have a file server with FTP on it.

File server: 192.168.55.2

I would like to redirect all incoming FTP requests on the firewall server to the file server. How do I do that?