What is Firebase Extensions and how do you use them?

Firebase Extensions are pre-packaged, open-source solutions that add functionality to your Firebase app with minimal code. They are managed, auto-updating Cloud Functions that respond to Firebase events. Popular extensions: (1) Trigger Email — monitors a Firestore collection for email documents and sends them via SendGrid/Mailchimp/SMTP; (2) Resize Images — automatically creates multiple resized versions of uploaded images to Firebase Storage; (3) Search with Algolia — syncs Firestore data to Algolia for full-text search automatically; (4) Delete User Data — deletes all Firestore, RTDB, and Storage data for a user when their account is deleted (GDPR compliance); (5) Stripe Payments — manages customer subscriptions and payments; (6) Translate Text — translates text in Firestore documents using Cloud Translation API. Install via CLI: firebase ext:install firebase/firestore-send-email. Extensions are configured with environment variables during installation and deployed as Cloud Functions in your project. They are open-source (GitHub: firebase/extensions) — you can inspect, fork, and self-host them.