Quick answer
Firebase Authentication - Firebase Authentication wins for most builders because its free tier is easier to start with and stays no-cost for more common auth use cases, while Auth0 is better when you need richer identity features and enterprise controls earlier.
How the free tiers compare
These products overlap on basic sign-in, but they diverge in what the free tier is trying to do. Firebase Authentication is the lighter lift for getting an app live: it has no-card free access, broad SDK support, and a generous no-cost allowance on Identity Platform MAUs, plus a $300 free credit if eligible on Blaze. Auth0’s free tier is more structured and feature-rich for identity work, especially around organizations, custom domains, passwordless, SCIM, and self-service SSO, but it is capped at 25,000 MAUs and explicitly requires credit card verification for custom domains. After free, Auth0 switches to flat-priced tiers starting at $35/month, then $240/month, which makes costs predictable but jumps faster. Firebase moves into usage-based pricing, so small apps can stay cheap, but costs depend on how much you use, especially SMS phone auth.
Auth0 vs Firebase Authentication free tier, side by side
| Auth0FTV 52 | Firebase AuthenticationFTV 62 | |
|---|---|---|
| Free MAUsFirebase also shows 50 SAML/OIDC monthly active users with Identity Platform. | Up to 25,000 monthly active users | Up to 50K monthly active users with Identity Platform |
| Custom domainsAuth0’s free plan says credit card verification is required for custom domains. | 1 custom domain | Not specified in the provided data |
| OrganizationsAuth0 Essentials raises this to 10. | 5 organizations | Not specified in the provided data |
| Enterprise connectionsThis is part of Auth0’s free-tier identity depth. | 1 enterprise connection | Not specified in the provided data |
| No-card free accessState this only because ftv_no_cc_required is explicitly present for both products. | No | Yes |
| Phone authFirebase’s free tier excludes phone number verification. | Included in free tier | Not included in the free tier; SMS is billed per message |
After you outgrow the free tier
Auth0 uses flat pricing after free, with Essentials at $35/month and Professional at $240/month before moving to contact sales for Enterprise. That makes the jump straightforward but sizable. Firebase Authentication moves to usage-based billing on Blaze, so many apps will pay little at first and more only as usage grows, especially for phone auth SMS. For small-team usage, Firebase is usually cheaper if you stay within no-cost limits; Auth0 is cheaper only if you specifically need the free-tier feature set and can justify the fixed monthly step up.
Cost at real usage
| Usage | Auth0 | Firebase Authentication |
|---|---|---|
| 25K MAUs/monthAuth0 free tier caps at 25K MAUs; Firebase no-cost Identity Platform usage is shown up to 50K MAUs. | Free (within tier) | Free (within no-cost usage) |
| 50K MAUs/monthFirebase’s no-cost Identity Platform allowance reaches 50K MAUs in the provided data. | At least $35/mo, since free caps at 25K MAUs | Free (within no-cost usage) |
| 1 phone auth SMSFirebase excludes phone number verification from the free tier. | Not specified in the provided data | Billed per SMS sent |
Estimates, not quotes. Usage-based rates change - verify with the vendor's pricing page before committing.
When to pick each one
Pick Auth0 when…
- You need stronger built-in identity features on the free tier, like SCIM, self-service SSO, or 5 organizations.
- You want passwordless auth and social connections without immediately moving to a paid plan.
- You are testing a product that needs a custom domain and can handle credit card verification.
- You expect to grow into enterprise-oriented controls like enterprise connections and hosted login customization.
- You prefer a predictable flat subscription once you pay, instead of metered usage billing.
Pick Firebase Authentication when…
- You want to start without a credit card on the free tier.
- You are building a small app and want the easiest path to basic email, social, or federated sign-in.
- You want to stay on a no-cost plan as long as possible before paying.
- You expect phone authentication to be a limited or optional part of the product and can pay per SMS when needed.
- You want a free tier that is simpler to adopt across web, mobile, and other platforms with minimal setup friction.
Bottom line
For most builders, Firebase Authentication is the easier free-tier choice because it has no-card entry and a more forgiving no-cost path for common app auth. Pick Auth0 if your app needs more identity structure from day one, especially organizations, SCIM, self-service SSO, or a more opinionated enterprise setup. If you are simply launching sign-in for a product and want to delay paid billing, Firebase is the safer default. If auth is part of a larger identity program, Auth0’s free tier is deeper, even if it is less forgiving.
Read the full listings: Auth0 and Firebase Authentication. Scores use the FTV methodology at /ftv. Browse more head-to-heads on /compare, or see the top-ranked free tiers on /top.