Firebase.database не является функцией
<script src="https://www.gstatic.com/firebasejs/7.24.0/firebase-auth.js"></script>
<script src="https://www.gstatic.com/firebasejs/7.24.0/firebase-database.js"></script>
// don't forget to change the "7.24.0" part of the url with the current version
Glorious Gnu