Yesterday, my collection on a another machine(running firefly) appeared fine in Amarok and I was able to play songs. Today I have nothing appear in Amarok, as if my other server does exists at all.

NOTE: Another machine running Windows with winamp and mt-daap plugin can still see my firefly collection.
NOTE: I've tried stopping and restarting the firefly and tested again from winamp, no problems, yet amarok still shows nothing.

I've not changed networking or firewall ( firewall is off actually ).

The machine with amarok is Kubuntu 9.04. It can ping my firefly server and I can connect to the web-admin from firefox.

Url details for last played song in my playlist in amarok is:
http://hel.local:3689/databases/1/items/9028.mp3
I can ping hel.local and it resolves to the correct machine ( 10.1.0.10 )
If I put this 'http://10.1.0.10:3689/databases/1/items/9028.mp3' in my firefox then it offers to save the file for me.
Yet amarok won't play the song or show the firefly collect in the collection area.

Any ideas?

Try starting amarok from console with -d appended to see, if amarok recognizes your firefly server.
And post the output here.
Be sure, that your firewall doesn't block port 3689 tcp/udp (I know, I know, but check it twice)

m0nk