MainApplication_android.java

// This will vary
package com.example.application;

// Core RNFirebase
import io.invertase.firebase.RNFirebasePackage;

public class MainApplication extends Application implements ReactApplication {

    @Override
    protected List<ReactPackage> getPackages() {
      return Arrays.<ReactPackage>asList(
          new MainReactPackage(),
          new RNFirebasePackage()  // Core
      );
    }
  };
  // ...
}