Zero-Point, "Limit local port range to:" and "Bind sockets to this address:" are exactly for what you mentioned. with "bind sockets" you tell flashfxp what's your inet ip address. and with "limit local port" you can configure firewall to forward some ports if you use NATP.
btw, check out
http://slacksite.com/other/ftp.html for a really good explanation of how PASV/PORT ftp connections work