"Powerful" maybe for the pub/sub capabilities that allow you to trigger actions on write and whatnot?

I don't know if Supabase has this, but at any rate I don't find anything else about Firestore "powerful", it's just a NoSQL database with all the pitfalls of NoSQL, a more limited API than MongoDB, a terrible data explorer UI, and very limited local capabilities...

I am. Firestore is powerful but it’s still a non-relational database, compared to the power of Postgres on the Supabase side

Not even close to as powerful as postgres.