Just having the "ffid=..." argument doesn't seem to be enough to make the owntone library appear in recent versions of Apple's Music.app; I copied the extra arguments from a real instance of Music.app and with them, the owntone library is usable in Music.app like it used to work in iTunes.
OwnTone
OwnTone is a media server that lets you play audio sources such as local files, Spotify, pipe input or internet radio to AirPlay 1 and 2 receivers, Chromecast receivers, Roku Soundbridge, a browser or the server’s own sound system. Or you can listen to your music via any client that supports mp3 streaming.
You control the server via a web interface, Apple Remote, an Android remote (e.g. Retune), an MPD client, json API or DACP.
OwnTone also serves local files via the Digital Audio Access Protocol (DAAP) to iTunes (Windows), Apple Music (macOS) and Rhythmbox (Linux), and via the Roku Server Protocol (RSP) to Roku devices.
Runs on Linux, BSD and macOS.
OwnTone was previously called forked-daapd, which again was a rewrite of mt-daapd (Firefly Media Server).
Looking for help?
Visit the OwnTone documentation for usage and set up instructions, API documentation, etc.
If you are looking for information on how to get and install OwnTone, then see the Installation instructions.