I had the exact same problem. Here is how I fixed it.
I uninstalled iTunes v.7.7.1, and installed v7.6.2. Here is how I did that;
1) drag the iTunes from the App folder to the trash.
2) open the system monitor (from the utilities folder) and find and stop the iTunes helper.
3) Empty the trash
4) copy and rename the iTunes folder from the Music folder to somewhere else
5) Download v7.6.2 from http://oldapps.com/download_iTunes_mac.php
6) Install it as usual
I suspect the new version of iTunes is somehow closing the socket when you click on a new song.
In my case, I could restart the firefly server process and click on a song in iTunes. That song and all subsequent songs in the shared library would play in succession with no problem, but if I double clicked another song, no matter what song it was, the firefly server would shut itself down with the OP error msg.
Until the error is fixed that is caused by the newest version of iTunes, just revert back to an earlier version.