title: how to get the top left coordinates of the screen with javascript on firefox
body: <p>I have a <code>&lt;div&gt;</code> element with <code>position: absolute</code> and z-index something big. I would like to cover the entire screen with this div with javascript.
  This is what I do and it works: </p>
  
  <pre><code>document.getElementById('mydiv').style.top = 0;
  document.getElementById('mydiv').style.left = 0;
  document.getElementById('mydiv').style.width = '100%';
  document.getElementById('mydiv').style.height = '100%';
  </code></pre>
  
  <p>However, when I scroll down with long webpages, this code displays my div at the top of the page, so the div renders above the region I currently view. 
  How can I change the top and left values so that my div always covers the active field I am viewing?</p>
  
  <p>I work on firefox 3.6.*.</p>
  
  <p>Thank you</p>