react-native-firebase/docs/faq/expo.md

Usage With Expo

If you use Expo and would like to use this package, you'll need to eject. If you do not want to eject, but wish to make use of features such as Realtime Database (without offline support) & Authentication, you can still use the Firebase Web SDK in your project.

For features such a Crash Reporting, Push Notifications (via Firebase not Expo), Performance Monitoring, AdMob, Analytics, Storage & Remote Config, you will have to eject your Expo project as these require the native SDKs, as the Web SDK does not support these.

You can see an indepth conversation here.