The reason Firefly isn’t actually starting is because it’s already running, or something else is using its port. Notice how it says that the address is already in use?
To double check, do a
ps waux | grep mt-daapdwhich will reveal any running instances
and/or
netstat -an | grep 3689which will show any usage of that port.
If you have made an init script, use
/etc/init.d/mt-daapd stopand try again. Otherwise,
sudo killall mt-daapdshould tidy things up.
I’m not sure about the Avahi-related error, but you have a problem with your ssc script plugin, which is not a major issue. It simply means that it won’t play if you try to serve a file which it thinks it should transcode.