title: Disable default alert sound for Firefox web notifications
body: <p>I'm using the <a href="https://developer.mozilla.org/en-US/docs/Web/API/Notification/Using_Web_Notifications" rel="noreferrer">Web Notification API</a> to show desktop notifications from my web app under OSX.</p>
  
  <p>In Firefox only, when a notification is triggered, it also plays a sound (in Safari and Chrome, using the exact same code, only the notification displays and there's no accompanying sound effect).</p>
  
  <p>I'd like to disable the notification sound in Firefox, but I can't find anything in the documentation about controlling it. Is this possible?</p>