No worries about the mp3 codec – it’s been a long way in Linux and still its not included in some distros by default for fear of patent infriction foobar….. 😉
What kind of playlist did you create?
Firefly does read m3u playlists, but they have to have the relative path, and haave to be put in the correct location on the server, obviously.
What’s more, you have to enable process_m3u’s and playlist_scan iirc in the firefly conf.