Comment by wilhuff

The backend is almost entirely custom, though many common low-level open-source components are used pervasively. For example, for components written in C++ we use Abseil, Google Test, etc.

The SDKs are all open source and make use of other open source components:

  * Android: https://github.com/firebase/firebase-android-sdk/tree/master/firebase-firestore
  * iOS: https://github.com/firebase/firebase-ios-sdk/tree/master/Firestore
  * Web: https://github.com/firebase/firebase-js-sdk/tree/master/packages/firestore

For example, the iOS SDK uses LevelDB for local key-value storage and gRPC for communication with the backend.