There’s always a thing called a log. (/var/log/mt-daapd.log)
Even if it doesn’t give sufficient information in the first place, you can raise the debuglevel up to 9.
If that gives you no clue, firefly had no trouble starting up.
Next guess would then be that the router does not have multicast (enabled).
Firefly relies on this to broadcast itself on the (wireless) network.