Firefly scans and indexes your music. It writes the info to a database. It cannot open the database, so firefly shuts itself down.
Make sure the user that runs firefly has read and write access to the database.
If that doesn’t work, maybe it’s corrupted. Then try deleting your database. (songs.db)
When you run firefly again, when no database is found, it does a full rescan and creates a new database from scratch.
Also, on startup, firefly scans for changes in your music. When the scan is done (without errors) the adminpage is launched.
Also, when you ran firefly in the foreground, it showed the log.
When running in the background, the log is written to a file.
Looking at your config file, it is written to /var/log/mt-daapd.log
Hope this helps