// firebase-messaging-sw.js
....
messaging.setBackgroundMessageHandler(function(payload) {
    console.log('[firebase-messaging-sw.js] Received background message ', payload);
    // Customize notification here
    let notificationTitle = 'Background Message Title';
    let notificationOptions = {
        body: 'Background Message body.',
        icon: 'resource/icon_web.png'
    };

    return self.registration.showNotification(notificationTitle, notificationOptions);
});

// popup.js
firebase.initializeApp(config);
const messaging = firebase.messaging();
messaging.onMessage(function (payload) {
  console.log('Message received. ', payload);
});

[firebase-messaging-sw.js] Received background message
{data: {…}, from: "153493200810", collapse_key: "do_not_collapse"}