Firefly media server broadcasts its presents in the local network.
The discovery of a shared iTunes library is done the same way.
When you make firefly ‘approachable’ using an SSH tunnel (assuming fizze is right, multicast does not go through tunnel) firefly does not broadcast through the tunnel.
What I have used in the past is Rendezvousproxy. I have used this little java application to see the firefly hosted at home from iTunes at school. http://ileech.sourceforge.net/index.php?content=RendezvousProxy-Download
(Note: using this alternative, you don’t need an SSH tunnel anymore)