title: In Firefox Developers Tools, in the Network Monitor, the tab Preview is always empty
body: <p>I'm using "Firefox Developers Tools" to debug my webapps. The tab Network Monitor show all request, and in each one I can see Headers, Cookies, Responses, Timing but the Preview tab not show the content, it's always empty, else even when the Response (from server) is an html.</p>
  
  <p><img src="https://i.stack.imgur.com/RuxGC.jpg" alt="the response of the request">
  <img src="https://i.stack.imgur.com/FFw4q.jpg" alt="the preview tab is empty"></p>
  
  <p>Who ever happened something similar?</p>