This addon for Firefox for Android sends a Firefox Desktop user agent when visiting YouTube Music. It allows you to stream music using the responsive UI on mobile, without making everything super-small with the desktop mode. It seems that Firefox can handle the "full" version of the site well enough that the extra functionality is beneficial.
This is the User Agent used:
Mozilla/5.0 (X11; Linux x86_64; rv:62.0) Gecko/20100101 Firefox/62.0
This add-on is available at Firefox Add-ons: https://addons.mozilla.org/en-US/firefox/addon/desktop-ua-on-yt-music-android/
This addon is based on Chrome UA for Google:
https://github.com/JonathanNakandala/Chrome-UA-for-Firefox-Android-on-Google
which in turn is based on this user-agent rewriter:
https://github.com/mdn/webextensions-examples/tree/master/user-agent-rewriter
It's based on the new WebExtensions standard.