using Firebase.Database;

private DatabaseReference db;

void start () {
    //jeito errado
    FirebaseDatabase.DefaultInstance.GetReference("/session/users/azul").ValueChanged += HandleUserStatus;

    //jeito certo
    db = FirebaseDatabase.DefaultInstance.GetReference("/session/users/azul");
    db.ValueChanged += HandleUserStatus;
}

private void HandleUserStatus(object sender, ValueChangedEventArgs args) {
       //sua função aqui!!!!
}

void OnDestroy () {
    db.ValueChanged -= HandleUserStatus;
}