Ok, so the wireless laptop thats serving up is running Ubuntu and the wired PC is running Windows XP, right?
The fact that not even banshee or rhythmbox are able to see firefly’s share on the same computer tells me there is something wrong there 😉
Can you access firefly’s web interface?
Did you add any songs, can you use the old applet?
http://localhost:3689/applet.html ? And do the songs play?
IF all thats working, likely avahi/bonjour is acting up.
iTunes should be able to see a rhytmbox or banshee DAAP share, but not vice versa. I guess it’s best to leave out firefly of the equation until you’ve set network things straight.