Firebase is not defined error when initializing firebase

I was following a tutorial and ReferenceError: firebase is not defined keeps poping up

I tried following some past question on stackOverflow but was not able to solve this. please tell me if i am doing something wrong

   <script src="https://www.gstatic.com/firebasejs/13.15.1/firebase-app.js"></script>
   <script src="https://www.gstatic.com/firebasejs/13.15.1/firebase-auth.js"></script>
   <script src="https://www.gstatic.com/firebasejs/13.15.1/firebase-firestore.js"></script>
   <script>
     // Initialize Firebase
     const firebaseConfig = {
        apiKey: "AIzaSyDdM1wifQDRC5Yx1lqiMbMXHhas_gQtgog",
        authDomain: "notes-project-e1952.firebaseapp.com",
        projectId: "notes-project-e1952",
        storageBucket: "notes-project-e1952.appspot.com",
        messagingSenderId: "730725354888",
        appId: "1:730725354888:web:b51688d6904jdvnjsb87e3d",
        measurementId: "G-4FJRM2XQW5"
        };

        // Initialize Firebase
        firebase.initializeApp(firebaseConfig);
   
        // Make auth and firestore references
        const auth = firebase.auth();
        const db = firebase.firestore();
   </script>
   
  <script src="https://cdnjs.cloudflare.com/ajax/libs/materialize/1.0.0/js/materialize.min.js"></script>
  <script src="scripts/auth.js"></script>
  <script src="scripts/index.js"></script>