title: Firebase Auth - Activating SSO login methods for existing users only
body: <p>I'd like to implement SSO logins for users in my Firebase web app, but only for users that have an existing account. When I add e.g. the Microsoft provider in Firebase and implement the required UX flows in the web app, anyone with a Microsoft account is able to sign in using that, upon which a new user account is created (if they haven't signed in before). I'd like to restrict this to users that:</p>
  <ul>
  <li>have registered previously with e.g. an email and password login method, and</li>
  <li>have subsequently chosen to activate the SSO login method in the web app</li>
  </ul>
  <p>Is there a recommended way to prevent the default behaviour in Firebase auth for SSO so that I can check for an existing account with the same email and its approved login methods before letting the user log in and/or create a new account?</p>