old_contents: // Import and configure the Firebase SDK
  // These scripts are made available when the app is served or deployed on Firebase Hosting
  // If you do not serve/host your project using Firebase Hosting see https://firebase.google.com/docs/web/setup
  importScripts('/__/firebase/8.2.5/firebase-app.js');
  importScripts('/__/firebase/8.2.5/firebase-messaging.js');
  importScripts('/__/firebase/init.js');
  
  const messaging = firebase.messaging();
  
  /**
   * Here is is the code snippet to initialize Firebase Messaging in the Service
   * Worker when your app is not hosted on Firebase Hosting.
  
   // Give the service worker access to Firebase Messaging.
   // Note that you can only use Firebase Messaging here. Other Firebase libraries
   // are not available in the service worker.
   importScripts('https://www.gstatic.com/firebasejs/8.2.5/firebase-app.js');
   importScripts('https://www.gstatic.com/firebasejs/8.2.5/firebase-messaging.js');
  
   // Initialize the Firebase app in the service worker by passing in
   // your app's Firebase config object.
   // https://firebase.google.com/docs/web/setup#config-object
   firebase.initializeApp({
     apiKey: 'api-key',
     authDomain: 'project-id.firebaseapp.com',
     databaseURL: 'https://project-id.firebaseio.com',
     projectId: 'project-id',
     storageBucket: 'project-id.appspot.com',
     messagingSenderId: 'sender-id',
     appId: 'app-id',
     measurementId: 'G-measurement-id',
   });
  
   // Retrieve an instance of Firebase Messaging so that it can handle background
   // messages.
   const messaging = firebase.messaging();
   **/
  
  
  // If you would like to customize notifications that are received in the
  // background (Web app is closed or not in browser focus) then you should
  // implement this optional method.
  messaging.onBackgroundMessage(function(payload) {
    console.log('[firebase-messaging-sw.js] Received background message ', payload);
    // Customize notification here
    const notificationTitle = 'Background Message Title';
    const notificationOptions = {
      body: 'Background Message body.',
      icon: '/firebase-logo.png'
    };
  
    self.registration.showNotification(notificationTitle,
      notificationOptions);
  });
  
new_contents: // Import and configure the Firebase SDK
  // These scripts are made available when the app is served or deployed on Firebase Hosting
  // If you do not serve/host your project using Firebase Hosting see https://firebase.google.com/docs/web/setup
  importScripts('/__/firebase/8.2.5/firebase-app.js');
  importScripts('/__/firebase/8.2.5/firebase-messaging.js');
  importScripts('/__/firebase/init.js');
  
  const messaging = firebase.messaging();
  
  /**
   * Here is is the code snippet to initialize Firebase Messaging in the Service
   * Worker when your app is not hosted on Firebase Hosting.
  
   // Give the service worker access to Firebase Messaging.
   // Note that you can only use Firebase Messaging here. Other Firebase libraries
   // are not available in the service worker.
   importScripts('https://www.gstatic.com/firebasejs/8.2.5/firebase-app.js');
   importScripts('https://www.gstatic.com/firebasejs/8.2.5/firebase-messaging.js');
  
   // Initialize the Firebase app in the service worker by passing in
   // your app's Firebase config object.
   // https://firebase.google.com/docs/web/setup#config-object
   firebase.initializeApp({
     apiKey: 'api-key',
     authDomain: 'project-id.firebaseapp.com',
     databaseURL: 'https://project-id.firebaseio.com',
     projectId: 'project-id',
     storageBucket: 'project-id.appspot.com',
     messagingSenderId: 'sender-id',
     appId: 'app-id',
     measurementId: 'G-measurement-id',
   });
  
   // Retrieve an instance of Firebase Messaging so that it can handle background
   // messages.
   const messaging = firebase.messaging();
   **/
  
  
  // If you would like to customize notifications that are received in the
  // background (Web app is closed or not in browser focus) then you should
  // implement this optional method.
  // Keep in mind that FCM will still show notification messages automatically 
  // and you should use data messages for custom notifications.
  // For more info see: 
  // https://firebase.google.com/docs/cloud-messaging/concept-options
  messaging.onBackgroundMessage(function(payload) {
    console.log('[firebase-messaging-sw.js] Received background message ', payload);
    // Customize notification here
    const notificationTitle = 'Background Message Title';
    const notificationOptions = {
      body: 'Background Message body.',
      icon: '/firebase-logo.png'
    };
  
    self.registration.showNotification(notificationTitle,
      notificationOptions);
  });
  
subject: Add notification customization info (#537)