Firebase identifies accounts by provider 1. Firebase identifies accounts by provider Firebase Auth doesn't rely solely on email address to identify a user. It associates each account with a *provider ID* (such as `password`, `google.com`, `facebook.com`). Therefore, the same email address can be linked to multiple accounts if used with different providers. 2. Facebook doesn't always share email When a user logs in via Facebook, Firebase retrieves information from their Facebook profile. If the user hasn't verified their email address on Facebook or if permissions are limited, Firebase may create an account without associating the email address, or with a different address (e.g., `facebook_123456@facebook.com`), which prevents automatic merging. 3. No automatic merging between providers Firebase doesn't automatically merge accounts created with different providers, even if the email address is the same. This prevents security conflicts and identity errors. The merging must be explicitly handled by the developer via calls to `linkWithCredential`.