To fix the “uncaught referenceerror firebase is not defined” error, you need to make sure that the Firebase library is properly imported in your project.

Here are the steps to do so:

1. Include the Firebase JavaScript library in your HTML file by adding the following script tag in the head section:

<script src="https://www.gstatic.com/firebasejs/7.16.1/firebase-app.js"></script>

2. Initialize the Firebase app by adding the following script code:

// Your web app's Firebase configuration
var firebaseConfig = {
apiKey: "YOUR_API_KEY",
authDomain: "YOUR_AUTH_DOMAIN",
databaseURL: "YOUR_DATABASE_URL",
projectId: "YOUR_PROJECT_ID",
storageBucket: "YOUR_STORAGE_BUCKET",
messagingSenderId: "YOUR_MESSAGING_SENDER_ID",
appId: "YOUR_APP_ID",
measurementId: "YOUR_MEASUREMENT_ID"
};
// Initialize Firebase
firebase.initializeApp(firebaseConfig);

3. Replace “YOUR_API_KEY” and other placeholder values with your own Firebase project’s configuration values. You can find these values in the Firebase console.
4. If you want to use a specific Firebase service (e.g. Firebase Realtime Database, Firebase Storage, etc.), you’ll need to import the corresponding JavaScript library. For example, to use the Realtime Database, you’ll need to add the following script tag:

<script src="https://www.gstatic.com/firebasejs/7.16.1/firebase-database.js"></script>

5. Finally, make sure that you are initializing the Firebase app after the Firebase JavaScript libraries are loaded.

I hope this helps! Let me know if you have any other questions.