Firebase is a large suite of products - are you talking about the Realtime Database? The answer is it depends on what "high scale" is. There are parts of the RTDB that didn't scale well when I worked on it (indexes are built in memory on demand so building the initial index is slow), but there are projects that could fix that.

Realtime Database? No. Firestore however, scales as most GCP services do and is it's in-house built replacement.