http-on-modify-response in Add-on SDK ff

const { Ci, Cu, Cc, Cr } = require('chrome');

Cu.import('resource://gre/modules/Services.jsm');

var observer = {
  observe: function(aSubject, aTopic, aData) {
    if (aTopic == 'http-on-modify-request') {
      console.log('we are in observer http-on-modify-request');
    } else if (aTopic == 'http-on-modify-response'){
      console.log('we are in observer http-on-modify-response');
    }

  }
};

console.log('!!!!!! start fine');

Services.obs.addObserver(observer, 'http-on-modify-response', false);
Services.obs.addObserver(observer, 'http-on-modify-request', false);

console.log('!!!!!! start better');

exports.onUnload = function (aData, aReason) {
  Services.obs.removeObserver(observer, 'http-on-modify-response');
  Services.obs.removeObserver(observer, 'http-on-modify-request');
};