title: How to center the label on the screen?
body: <p>I'm specifing the size of the windows within the window.open(),
  but regardless of the size, I would like this label to be exactly on the center of the screen (vertically and horizontally):</p>
  
  <pre><code>&lt;?xml version="1.0" encoding="UTF-8"?&gt;
  
  &lt;window sizemode="maximized" hidechrome="true" id="weatherWindow" title="Window"
      xmlns="http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul"&gt;
  
      &lt;label style="background:#FFFF00; margin: auto auto auto auto; text-align:center;"&gt;LOADING...&lt;/label&gt;
  
  &lt;/window&gt;
  </code></pre>
  
  <p>The text is centered horizontally only, any idea how to center it vertically either?</p>