title: Firefox not showing AJAX requests from a content script in the "Network" tab
body: <p>I'm trying to build my first webextension, but having some troubles regarding web requests. When I do an AJAX request (or, indeed, any request at all), it doesn't show up in the "Network" tab.</p>
  
  <p>I'm looking at both the network tab of the application, and the network tab in the addon debug window.</p>
  
  <pre><code>$.ajax({
          url: 'https://steamcommunity.com/market/sellitem/',
          type: 'POST',
          data: {
              // data here
          },
          crossDomain: true,
          xhrFields: { withCredentials: true }
      }).done(function (data) {
          // handler here
      }).fail(function (jqxhr) {
          // handler here
      });
  </code></pre>
  
  <p>I can see the request ends in 400 if I hit a breakpoint in "fail" branch and inspect the jqxhr variable, but I would really like to be able to compare the requests in the network tab to find out what goes wrong.</p>
  
  <p>Edit 1: My permissions in the manifest.json:</p>
  
  <pre><code>"permissions": [
          "*://steamcommunity.com/market/*",
          "webRequest"
      ],
  </code></pre>