FireFly is a third party iTunes streaming server which many users have installed on their WHS systems. It's also installed and configured by Hewlett-Packard on their HP MediaSmart servers.
Installation is pretty simple. You download the installation package, log on to your server directly (through remote desktop or physical console), and install it. Then you configure it to look (usually) at the Music share on your server, and copy all your iTunes tracks to that location. See the FireFly forums for more information about configuring it.
I'm not on the WHS team, I just post a lot. :)