index.html

...
<script src="https://www.gstatic.com/firebasejs/7.19.0/firebase-app.js"></script>
<script src="https://www.gstatic.com/firebasejs/7.19.0/firebase-firestore.js"></script>

<script>
  // Your web app's Firebase configuration
  var firebaseConfig = {
    ...
  }
  // Initialize Firebase
  firebase.initializeApp(firebaseConfig)
  const db = firebase.firestore()
</script>

<script src="./index.js"></script>
...