Please note v5 is still seeing releases, but they are for breaking ecosystem changes only - just enough so it remains relevant for those that can't move to v6 yet.
This change allows use of Sign-in with Apple, an iOS13+ requirement to avoid rejection by the App Store if you use any forms of social authentication.
Breaking: Note that iOS13+ is only supported with Xcode 11+, which requires react-native 0.60+
You may find that react-native-firebase v5.6.x still works on react-native 0.59 (the test suite still does), but you will not be able to integrate Sign-in with Apple until you upgrade as that library is react-native 0.60+ only.
It works well with https://github.com/invertase/react-native-apple-authentication including a guide specifically for integrating with react-native-firebase, though note the import statements are for v6, you'll need to import auth in the v5 style to use it: https://github.com/invertase/react-native-apple-authentication/blob/master/docs/FIREBASE.md
See discussion in the related issue: #2884