title: Ajax file upload progress event not firing
body: <p>I am uploading a file via ajax request, by simply splitting them in to chunks.</p>
  
  <p>The problem is progress event, Firefox for some reason doesn't want to fire that event, here is my code (most of the unnecessary code is removed)</p>
  
  <pre><code>//slice file
  if(file.mozSlice){
      chunk   = file.mozSlice(startByte, endByte);
  }else if(file.slice){
      chunk   = file.slice(startByte, endByte);
  }else{
      chunk   = file;
      isLast  = true;
  }
  
  
  var xhr = new XMLHttpRequest();
  
  xhr.upload.addEventListener('progress', function(e){
      console.log('progress');
  }, false);
  
  xhr.upload.addEventListener('error', function(e){
      console.log("upload error!");
  });
  
  xhr.onreadystatechange = function(e){
      if(this.readyState == 4 &amp;&amp; this.status == 200){
          //this chunk has bee uploaded, proceed with the next one...
      }
  }
  
  xhr.open('POST', "", true);
  
  xhr.setRequestHeader('Cache-Control', 'no-cache');
  xhr.setRequestHeader('X-Requested-With', 'XMLHttpRequest');//header
  xhr.setRequestHeader('Content-Type', 'application/octet-stream');//generic stream header
  xhr.send(chunk);
  </code></pre>
  
  <p>I'm sure i haven't made any big mistakes since chrome works without any problems, so there must be some Firefox related issue.</p>