I just installed firefly server for my user (you don’t have to share it for everyone), and I hardcoded in all the paths in the firefly.conf file.
Then I create a FireflyService.plist file in /Library/LaunchDaemons (to make this run as root). The contents are:
Where startup_script.sh contains nothing more than:
/path.to.firefly/firefly -c /path.to.firefly/firefly.conf
Hope this helps!