Supabase Auth is very similar to Firebase Auth. Both are easy to setup with just a couple lines of code. The advantage, imo, is that Supabase just stores your user data in a regular Postgres database so you can then set up relations with other tables.

I haven't used Flutter, but it looks like Supabase has a flutter library and documentation on how to use it: https://supabase.com/docs/guides/getting-started/tutorials/w...

I used Firebase for a long time, and Supabase more recently. I have no desire to go back to Firebase. No longer having to obsess over read/write count (like you do with Firestore), and being able to leverage full SQL when needed, is a game changer.