To open the limewire (gnutella) port do the following as root:
iptables -I INPUT -p tcp --dport 6346 -j ACCEPT
iptables -I INPUT -p udp --dport 6346 -j ACCEPT
service iptables save
It's been awhile since I've used limewire, so you should check the limewire docs and verify that the port number is correct. If you aren't comfortable with the command line, the use the firewall config GUI in Start->System Settings->Security Level and enter the appropriate port number in the "other ports" box.