When a new email/password user is created, in order to verify their email, need to send a verification email. Firebase has built in support for this which needs to be exposed in firestack.

Android: https://firebase.google.com/docs/auth/android/manage-users#send_a_user_a_verification_email

iOS: https://firebase.google.com/docs/auth/ios/manage-users#send_a_user_a_verification_email

Web: https://firebase.google.com/docs/auth/web/manage-users#send_a_user_a_verification_email