This is quite normal. The ports in the wiki posting are outgoing ports. The incomming ports are negotiated while establishing the connections and will move every time you establish a new connection.to the SL and voice servers. This is how IP works.
Your firewall should support a method to open the incomming port automatically based on this negotiation. With my linux based firewall this is done with
iptables -A FORWARD -i $DSL -o $LAN -m state --state ESTABLISHED,RELATED -j ACCEPT iptables -A FORWARD -o $DSL -i $LAN -m state --state ESTABLISHED,RELATED -j ACCEPT
The "--state ESTABLISHED,RELATED" says: If there is an outgoing connection established, open the related incomming port, which is negotiated while establishing the connection.
I don't know how your Firewall is configured, but there should be such an option. Most Connections to the internet need it.