This seems backwards to me … but probably because I did it the way I explain below.
Try stopping Firefly and getting iTunes to work first. You should be able to tell iTunes to look at your WHS machine for the files (in iTunes, use Edit > Preferences > Advanced > General and edit path for “iTunes Music Folder Location”.
Once you can get iTunes to recognize the network shared drive and play songs from it, then activate Firefly and configure it (if you want to be able to play music without going through iTunes, that is).