There have been several projects from fanatics who have created different solutions to reach firefly from outside your local network.
Ofcourse, all these applications require the port to be forwarded.
– Fireplay; A browser-based flash application, which can search music and instantly stream it.
– DaapPlaylistGenerator; An application which generates a playlist of the complete database known by firefly, and outputs it into a .m3u playlist file. This is practically a playlist of streams, which can be accessed from anywhere on the net.
These applications can be found in the Add-On software forum.
Another way is by using RendezvousProxy to forward the local port to the remote port of firefly, which makes your comp think he is local with firefly. The shared library then shows up in iTunes.