How To Handle Multiple Sign In Methods For The Same User In Firebase?

I'm currently developing an authentication system with Firebase. I'd like my system to accept email/password, Google and Facebook as sign-up and sign-in methods. So far, so good. E

Solution 1:

One option is to enforce password users to verify their email address right after they sign up. In the example #2, Firebase will keep the account's existing password if the email address has been verified e.g. by sending a verification link to the email address and the user has clicked the link.