Firebase Auth may delete old (30 days) anonymous users if Auto clean-up is enabled. So when a user's app will call
Firebase.auth.signInAnonymously()
it returns new anonymous User ID
https://firebase.google.com/docs/auth/web/anonymous-auth?authuser=0&_gl=1*bvz710*_ga*MTM2NDY5NDY3Ni4xNzIwNDM0OTky*_ga_CW55HF8NVT*czE3NjQ3NTc0NzIkbzY2MyRnMSR0MTc2NDc1NzQ4OCRqNDQkbDAkaDA.&hl=en#auto-cleanup