When you add Plugin.CloudFirestore package, FirebaesAuth.Instance property is always null.
If you delete this package, FirebaseAuth.Instance object exists.
My development environment is as follows.
macOS 10.15
Visual Studio for Mac Community 8.3.4
Xamarin Forms + Prism.Forms + Prism.Unity.Forms + Xamarin.Firebase.Auth
When you add Plugin.CloudFirestore package, FirebaesAuth.Instance property is always null. If you delete this package, FirebaseAuth.Instance object exists.