title: Jquery animate bug in firefox
body: <p>I use this code, but it's like the offset is never good. 
  The goal of the offset:  my menu bar stay on top. So when i click on menu item it's scroll to top. minus the height of the menu. But  i always have a glitch that scroll again . </p>
  
  <p>works fine in chrome but not in firefox an IE. is it clear?</p>
  
  <pre><code> $('html, body').animate({
         scrollTop: $(this.hash).offset().top - menu.offsetHeight
       }, 300, function(){
         window.location.hash = hash;
       });
  </code></pre>