Just add that drive to the /etc/fstab, and it will be mounted at boot.
Or, just add the mount command to firefly’s init script.
There should be a init script that starts firefly automatically at boot. My guess is that it already is in place, but fails because it cannot access the music directory, and thus bails. 😉
Both topics are already well documented, and while the former is common amongst Unices, the latter is a Ubuntu specific issue.