How would you like to use Firebase Authentication from your .Net application specifically? Do you want to generate a custom token which can be used to sign in a client device? If so, you can use a third-party JWT library to do that, documented
here. Do you want to read from the Realtime Database from .NET? If so, you can use
the REST API. Do you want to verify an ID token from a client device? If so, you can use a third-party JWT library to do that, documented
here. Do you want to create or manage Firebase Auth users? Unfortunately, that is not currently possible from a .NET application. Do you want to do something else? Let me know what it is and I'll try to help you out.