Ripped from an older KB, but I believe it still applies...
enable both incoming and outgoing TCP connections to the 6891-6900 range of TCP ports. This allows each sender to perform up to 10 simultaneous file transfers. Note that if only TCP port 6891 is open, users are only able to perform one file transfer at a time
v6.0 supposedly works better for FT thru routers, NATs, firewalls, etc, whereas you don't need these ports open.