Supabase is the open-source alternative to Firebase. Like Firebase, it's a complete app development platform with user authentication, cloud functions, APIs, Postgres database, storage, vector embeddings, and other features.
Supabase projects come with PostgreSQL's policy engine for fine-grained user access rules. It has social login integrations with Google, Facebook, GitHub, Azure (Microsoft), Gitlab, Twitter, Discord, and many more.
They offer a customizable authentication component for React.
Phone login and MFA can be added through third-party SMS providers such as Twilio or Bird.
The free tier comes with 50,000 monthly active users. The Pro plan, at $25/month, comes with 100,000 MAUs and $0.00325/MAU beyond that.