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 monthly active users, then pay-as-you-go at $0.0055 per MAU for the next band (50,001 to 100,000), dropping in later tiers as volume grows. Phone auth (SMS) is billed per message even on the free tier, roughly $0.01 in the US, Canada, and India and as much as $0.06 in other regions. For web apps using email and social login only, the cost difference shows up just past 50,000 MAU.

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 ships with more than 80 providers preconfigured: 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.

Maintenance trajectory. This is the one current factor that did not exist when most Firebase-vs-AuthJS comparisons were written. In September 2025 the Auth.js project announced it is now part of Better Auth. Existing Auth.js installs keep getting security and critical fixes, but the maintainers now recommend Better Auth for new projects. AuthJS is not abandoned, and the stable next-auth v4 line still ships, yet the headline v5 release has sat in beta for years rather than reaching a stable tag. Firebase Auth, by contrast, is a funded Google Cloud product with a continuous release cadence. If long-horizon maintenance is a deciding factor for you, weigh that the most actively-growing path in the AuthJS family now points at a different package name.

By the Numbers (2026)

The voice-of-experience sections above are timeless. The figures below are not, so here is exactly where they stand as of May 2026, with sources at the end of the post.

AuthJS (next-auth):

• Latest stable release is next-auth 4.24.14, published 2026-04-14. The next-generation v5 line is still pre-release at 5.0.0-beta.31, and its companion @auth/core package sits at 0.34.3.
• 28,259 GitHub stars and 4,043 forks. Written in TypeScript, distributed under the permissive ISC license.
• About 4.24 million weekly npm downloads for next-auth, plus another 3.09 million for @auth/core, in the week ending 2026-05-27. That is the adoption gap behind the "everybody-uses-it" reputation.
• 80+ OAuth providers preconfigured out of the box.
• Cost: $0, with no MAU ceiling, no per-seat charge, and no SMS surcharge, because it runs inside your own app.

Firebase Auth:

• Free up to 50,000 monthly active users for standard email/password and social sign-in, on both the no-cost Spark plan and the pay-as-you-go Blaze plan.
• Above 50,000 MAU the graduated Google Cloud Identity Platform rate kicks in: $0.0055/MAU for 50,001 to 100,000, $0.0046/MAU for 100,001 to 1,000,000, $0.0032/MAU for 1,000,001 to 10,000,000, and $0.0025/MAU beyond that.
• Enterprise SAML and OIDC sign-in is free for the first 50 MAU, then $0.015/MAU.
• Phone and SMS verification is billed per message regardless of MAU tier, roughly $0.01 in the US, Canada, and India and up to about $0.06 in many other regions.
• First-class native SDKs for iOS, Android, Flutter, and Unity, which AuthJS does not offer at all.

Real Cost at Solo-Dev Scale

Pricing pages quote per-unit rates, but the number that actually matters is what hits your card at the scale you are realistically at. Here is a worked example with the rates above, so you can swap in your own assumptions.

Assumptions. A solo-dev SaaS that has done well: 75,000 monthly active users, email and Google sign-in only (no phone or SMS), no enterprise SAML. We compare a full month on each option.

AuthJS: $0 in auth fees. The library runs inside your existing app server, so the only marginal cost is the compute you were already paying for. At 75,000 MAU that is still $0 attributable to authentication.

Firebase Auth: the first 50,000 MAU are free. The remaining 25,000 fall in the 50,001 to 100,000 band at $0.0055/MAU. That is 25,000 x $0.0055, which is $137.50 for the month, or about $1,650 for the year. Add nothing for SMS in this scenario because we assumed email and social only. Turn on phone verification and each code sent adds roughly a cent or more on top.

The read. Below 50,000 MAU the two are a tie at $0 for a web app, so cost should not drive the decision there. The crossover only appears once you push past the free tier, and even then $137.50 a month at 75,000 active users is not the line item that sinks a solo SaaS. Choose on platform fit, mobile support, and the maintenance trajectory above, not on this number, unless you are forecasting hundreds of thousands of MAU. If you are, the per-MAU rate keeps falling as you scale, which works in Firebase's favor at the very top end.

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.

Sources

All figures above were checked on 2026-05-28.

• Firebase pricing, free tier and plan structure: https://firebase.google.com/pricing
• Google Cloud Identity Platform pricing page: https://cloud.google.com/identity-platform/pricing
• Graduated per-MAU bands, SAML/OIDC rate, and SMS per-message detail: https://www.metacto.com/blogs/the-complete-guide-to-firebase-auth-costs-setup-integration-and-maintenance
• next-auth versions and weekly downloads, npm registry and downloads API: https://registry.npmjs.org/next-auth and https://api.npmjs.org/downloads/point/last-week/next-auth
• next-auth on npm: https://www.npmjs.com/package/next-auth
• next-auth GitHub repository, stars, forks, language, license: https://github.com/nextauthjs/next-auth
• Auth.js OAuth providers (80+ preconfigured): https://authjs.dev/getting-started/authentication/oauth
• Auth.js v5 migration status: https://authjs.dev/getting-started/migrating-to-v5
• Auth.js is now part of Better Auth, maintainer announcement: https://github.com/nextauthjs/next-auth/discussions/13252