title: checkbox returns "on" value after it is unchecked
body: <p>I'm following a tutorial where you make a checkbox that enables a disabled button and when you uncheck the box, the button becomes disabled again. The problem is that when I uncheck the box, it seems like the checkbox still in an "on" state because the button is not disabled. The code works in Chrome but not in Firefox 3.6.</p>
  
  <p>Here is my code:</p>
  
  <pre><code>&lt;p&gt;&lt;input type="checkbox" id="agree" &gt;I agree&lt;/p&gt;
  &lt;input id="continue" type="button" value="continue" disabled='true'&gt;
  
  
  &lt;script&gt;
          $('#agree').change(function(){
         var state = $(this).attr('value');
          if(state == 'on'){
          $('#continue').removeAttr('disabled');
          }else if(state == ''){
              $('#continue').attr('disabled', 'false');
          }
          });
      &lt;/script&gt;
  </code></pre>