title: Firefox 4 onBeforeUnload custom message
body: <p>In Firefox <strong>3</strong>, I was able to write a custom confirmation popup with:</p>
  
  <pre><code>window.onbeforeunload = function() {
     if (someCondition) {
        return 'Your stream will be turned off';
     }
  }
  </code></pre>
  
  <p>Now in Firefox <strong>4</strong>, it does not show my custom message. The default message that it provides is not even accurate to what my application does.</p>
  
  <p><img src="https://i.stack.imgur.com/4gaoU.png" alt="firefox 4 confirm"></p>
  
  <p>Can this default message be overridden?</p>