Alright, let's get started by adding the Firebase SDK to your app. You can grab it from the Firebase console on their website. It's like getting the secret sauce for your app! Once you've got the SDK, it's time to initialize Firebase in your app. You'll need to create a configuration object with details like your Project ID and API Key. Then, just call `firebase.initializeApp(config);` and you're good to go.

   // Your web app's Firebase configuration
  var firebaseConfig = {
    apiKey: "api-key",
    authDomain: "project-id.firebaseapp.com",
    projectId: "project-id",
    storageBucket: "project-id.appspot.com"
  };
  // Initialize Firebase
  firebase.initializeApp(firebaseConfig);

Now that Firebase is all set up, let's move on to authenticating your app using JWT tokens. First, you'll need to sign up or sign in a user with their email and password using `firebase.auth().signInWithEmailAndPassword(email, password)`.

   firebase.auth().signInWithEmailAndPassword(email, password)
    .then((userCredential) => {
      // Signed in 
      var user = userCredential.user;
    })
    .catch((error) => {
      var errorCode = error.code;
      var errorMessage = error.message;
    });

Once the user is authenticated, Firebase will automatically create a JWT token for their session. You can get this token by calling `firebase.auth().currentUser.getIdToken(true);` - the `true` parameter forces a token refresh.

   firebase.auth().currentUser.getIdToken(true)
  .then(function(idToken) {
    // Send token to your backend via HTTPS
  })
  .catch(function(error) {
    // Handle error
  });

With the JWT token in hand, you can now secure your endpoints. Whenever a user makes a request, they need to include the JWT token in the authorization header. On your server, you'll decrypt the JWT token to verify if the user has the right permissions to access that endpoint. On your server, you'll need to verify the JWT token provided in the request. You can use the Firebase Admin SDK or any JWT library if the Firebase Admin SDK isn't available. Once the token is verified, it means the request is from an authenticated user.

Just a heads-up: This is a simplified explanation. In a real-world scenario, you'll need to handle things like token expiration and user logout. Also, make sure your tokens are stored securely on the client side, like in HTTP-only cookies, to prevent XSS attacks.