title: Persistent storage maximum size reached while the length of storage is 0
body: <p>I got a "Persistent storage maximum size reached" error when I call the following code:</p>
  
  <pre><code>    localStorage.setItem('key', 1);
  </code></pre>
  
  <p>But when I get the length attribute of the localStorage, the result is 0:</p>
  
  <pre><code>    localStorage.length; // returns 0
  </code></pre>
  
  <p>The problem occurred on my Firefox 13.0.1 Mac version</p>