Firebase Auth vs AuthJS for Solo Developers

Quick Comparison

Firebase Auth Overview

Firebase Auth is Google's authentication service, part of the Firebase platform. It supports email/password, phone verification, anonymous auth, and social providers. The SDKs manage the entire flow automatically: token generation, refresh, session persistence, and user storage. You call a function and authentication happens.

The free tier covers 50,000 monthly active users. Firebase Auth integrates with Firestore security rules, Cloud Functions triggers, and the broader Google Cloud ecosystem. Mobile SDKs for iOS, Android, and Flutter handle platform-specific OAuth flows natively.

I've used Firebase Auth in projects where the backend was entirely Firebase. The integration between auth, Firestore rules, and Cloud Functions made the whole stack feel cohesive. Authentication was done in 20 minutes.

AuthJS Overview

AuthJS (formerly NextAuth.js) is an open-source authentication library for JavaScript frameworks. It supports Next.js, SvelteKit, and Express with official adapters. You configure OAuth providers, define callbacks for customizing tokens and sessions, and AuthJS handles the OAuth dance, CSRF protection, and session cookies.

AuthJS is free with no usage limits. It runs in your application, not on an external server. Add providers (Google, GitHub, Discord, email), optionally connect a database adapter for persistent sessions, and auth works. The library manages the complexity of OAuth flows while giving you hooks to customize behavior.

I've used AuthJS in several Next.js projects. Configuring Google and GitHub sign-in takes about 30 minutes. The session hook provides user data on every page. For web apps needing social login, AuthJS is fast and reliable.

Key Differences

Platform scope. Firebase Auth is part of a full backend platform. When you choose Firebase Auth, you're likely choosing Firestore, Cloud Functions, and Cloud Storage too. AuthJS is just an auth library. It doesn't care about your backend or database. You can use AuthJS with any backend: Supabase, PlanetScale, your own PostgreSQL, or no database at all (JWT-only sessions).

Mobile support. Firebase Auth has first-class native SDKs for iOS, Android, Flutter, and Unity. AuthJS is web-only (Node.js runtime). If you're building a mobile app, Firebase Auth has you covered. AuthJS doesn't work natively on mobile, though you could use it behind an API.

Cost. Both are free for most solo developers. AuthJS is free forever with zero limits. Firebase Auth is free up to 50,000 MAU, then pay-as-you-go. Phone auth (SMS) costs money even on the free tier. For web apps, the cost difference only shows up at significant scale.

Vendor dependency. AuthJS runs in your codebase. Auth works as long as your server works. Firebase Auth depends on Google's infrastructure. If Firebase has an outage, your users can't authenticate. AuthJS gives you independence. Firebase gives you reliability backed by Google's SRE team.

OAuth provider breadth. AuthJS supports over 60 OAuth providers out of the box: Google, GitHub, Discord, Twitch, Spotify, LinkedIn, and many more. Firebase Auth supports about a dozen major providers. If you need to authenticate users from a less common provider, AuthJS likely has an adapter. Firebase might not.

Session management. AuthJS gives you callbacks at sign-in, JWT creation, and session generation. You can add custom claims, modify tokens, and control session behavior in your code. Firebase Auth manages sessions through its SDK with less customization. Token refresh and persistence are automatic but opaque.

Anonymous auth. Firebase Auth supports anonymous accounts that convert to permanent accounts later. This is valuable for mobile apps where you want friction-free onboarding. AuthJS doesn't have built-in anonymous auth.

When to Choose Firebase Auth

• You're building a mobile app (native SDKs for iOS, Android, Flutter)
• You're using Firebase for your backend (Firestore, Cloud Functions)
• Anonymous auth for gradual onboarding is useful
• You want Google-backed infrastructure reliability
• Phone number verification is a requirement

When to Choose AuthJS

• You're building with Next.js or SvelteKit and want web-focused auth
• You need more than 12 OAuth providers (AuthJS supports 60+)
• You want zero vendor dependency for authentication
• Session customization and callback hooks matter
• You want to use any backend or database you choose

The Verdict

For web-only projects using Next.js or SvelteKit, AuthJS is the better fit. It's free, supports more OAuth providers, and gives you more control over sessions. You're not tied to any backend platform, and the library is flexible enough to work with whatever stack you've chosen.

For mobile projects or projects using the Firebase ecosystem, Firebase Auth is the clear winner. The native SDKs, anonymous auth, and platform integration make it the practical choice. You'll have auth working on iOS and Android without building an API layer.

My recommendation: let your platform guide the decision. Building a web app with Next.js? Use AuthJS. Building a mobile app or already using Firebase? Use Firebase Auth. If you're building both web and mobile and need shared auth, Firebase Auth provides the cross-platform consistency, but you could also put AuthJS behind an API that mobile clients call. Neither choice is wrong. They're optimized for different deployment targets.