Firestore のデータの読み込みがうまくいかない

猫の足跡というサイトをもとに開発をしています。
下記のコードでfirestoreのデータの読み込みができるはずですが、うまくいきません。なぜでしょうか。

html
1<script src="https://www.gstatic.com/firebasejs/7.6.1/firebase-app.js"></script>
2<script src="https://www.gstatic.com/firebasejs/5.8.1/firebase-app.js"></script>
3<script src="https://www.gstatic.com/firebasejs/5.8.1/firebase-firestore.js"></script>
4<!-- TODO: Add SDKs for Firebase products that you want to use
5     https://firebase.google.com/docs/web/setup#available-libraries -->
6<script src="https://www.gstatic.com/firebasejs/7.6.1/firebase-analytics.js"></script>
7
8<script>
9  // Your web app's Firebase configuration
10  var firebaseConfig = {
11    apiKey: "xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx",
12    authDomain: "xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx",
13    databaseURL: "xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx",
14    projectId: "xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx",
15    storageBucket: "xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx",
16    messagingSenderId: "xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx",
17    appId: "xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx",
18    measurementId: "xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx"
19  };
20  // Initialize Firebase
21  firebase.initializeApp(firebaseConfig);
22</script>

js
1var db = firebase.firestore();
2db.collection("users").get().then((query) => {
3  var buff = [];
4  query.forEach((doc) => {
5    var data = doc.data();
6    buff.push([doc.id, data.name, data.age]);
7  });
8  console.log(buff);
9})
10.catch((error)=>{
11  console.log(`データの取得に失敗しました (${error})`);
12});