Have you done this check?
@Firefly Wiki wrote:
On unix, check that you don’t have multiple mDNS daemons running. Sometimes, multiple mDNS responders, like the one built in to Firefly conflict with the ones you may already have installed (Apple’s implmentation, Howl, or Avahi). If you already have an mDNS responder, start mt-daapd with the “-m” argument to disable the internal mDNS daemon and edit the configuration for your existing mDNS daemon to advertise _daap._tcp on 3689 (or whatever port you are using) and _rsp._tcp on the same port.