Firebase will not call your onMessageReceived when your app is in background or killed, and you can’t customise your notification. System generated notification will show.

to make firebase library to call your onMessageReived in every cases

a) Foreground

b) Background

c) Killed

you must not put json key “notification” in your request to firebase api but instead use “data”, see below.

For example following message will not call onMessagereceived()

  {
  "to": "/topics/test",
  "notification": {
    "title" : "title",
    "message": "data!"
   }
}

but this will work

  {
  "to": "/topics/test",
   "data": {
       "title":"title",
       "message":"data!"
   }
} 

see this link:https://firebase.google.com/docs/cloud-messaging/concept-options it has a detailed description of firebase message type For example:

@Override
public void onMessageReceived(RemoteMessage remoteMessage) {

    Log.d(TAG, "From: " + remoteMessage.getFrom());

    // Check if message contains a data payload.
    if (remoteMessage.getData().size() > 0) {
        Log.d(TAG, "Message data payload: " + remoteMessage.getData());
        sendNotification(remoteMessage.getData().get("message").toString(), remoteMessage.getData().get("title").toString());
    }

}

    private void sendNotification(String message, String title) {
        int requestID = (int) System.currentTimeMillis();
        Intent intent = new Intent(this, activityCompat);
        
        PendingIntent pendingIntent = PendingIntent.getActivity(this, requestID, intent, PendingIntent.FLAG_UPDATE_CURRENT);

        NotificationCompat.Builder notificationBuilder = new NotificationCompat.Builder(this)
                .setSmallIcon(R.drawable.small_logo)
                .setContentTitle(title)
                .setContentText(message).setContentIntent(pendingIntent)
                .setAutoCancel(true)
                .setStyle(new NotificationCompat.BigTextStyle()
                        .bigText(messageBody))
                .setTicker(messageBody);

        NotificationManager notificationManager = (NotificationManager) getSystemService(Context.NOTIFICATION_SERVICE);
        notificationBuilder.getNotification().flags |= Notification.FLAG_AUTO_CANCEL;
        Notification notification = notificationBuilder.build();

        notificationManager.notify((int) Calendar.getInstance().getTimeInMillis(), notification);

    }