title: Firefox 'Cross-Origin Request Blocked' despite headers
body: <p>I'm trying to make a simple cross-origin request, and Firefox is consistently blocking it with this error: </p>
  
  <blockquote>
    <p>Cross-Origin Request Blocked: The Same Origin Policy disallows reading the remote resource at [url]. This can be fixed by moving the resource to the same domain or enabling CORS. [url] </p>
  </blockquote>
  
  <p>It works fine in Chrome and Safari. </p>
  
  <p>As far as I can tell I've set all the correct headers on my PHP to allow this to work. Here's what my server is responding with </p>
  
  <pre><code>HTTP/1.1 200 OK
  Date: Mon, 23 Jun 2014 17:15:20 GMT
  Server: Apache/2.2.22 (Debian)
  X-Powered-By: PHP/5.4.4-14+deb7u8
  Access-Control-Allow-Origin: *
  Access-Control-Allow-Methods: GET, POST, OPTIONS
  Access-Control-Allow-Headers: Content-Type
  Access-Control-Request-Headers: X-Requested-With, accept, content-type
  Vary: Accept-Encoding
  Content-Length: 186
  Content-Type: text/html
  </code></pre>
  
  <p>I've tried using Angular, jQuery, and a basic XMLHTTPRequest object, like so:</p>
  
  <pre><code>var data = "id=1234"
  var request = new XMLHttpRequest({mozSystem: true})
  request.onload = onSuccess;
  request.open('GET', 'https://myurl.com' + '?' + data, true)
  request.setRequestHeader('Content-Type', 'application/x-www-form-urlencoded')
  request.send()
  </code></pre>
  
  <p>...and it works in every browser except  Firefox. Can anyone help with this?</p>