title: window.postMessage fires the message event twice after upgrading firefox version 68.0 or beta version also
body: <p>when I fire window.postMessage method in firefox 68.0v or beta version only once, but this message is triggering twice in the firefox extension </p>
  
  <p>window.postMessage( { 'type':'sbCheckExtension'}, '*' );</p>
  
  <p>It should fire only once</p>