Yes, multicast is set.
I tried the Avahi-discover and there’s nothing except my linux computer’s services.
Firefly appeared when I configured Avahi as you suggested.
The bottom line is that tcpdump shows multicast packets from the SoundBridge destined for port 1900. But no other ports.
So, either UPnP is required for Firefly to work, or every other multicast packet the SoundBridge is sending is not getting to the server.