First drag event missed with ui.droppable.js file (only Firefox 3)

Hello everyone ! :)

Sorry for my English, I'm French.

I discover a bug with the lastest versions of ui.draggable (revision 563) and ui.droppable (revision 561) from the trunk branch, only on Firefox 3.

I created a list ul with li elements and added a draggable functionality to them with a custom div helper :

<script type="text/javascript">
  $(function(){
    $("li").draggable({
      cursorAt : { bottom : 5, right : 5 },
      helper   : function(){
        return $("#div").clone().html($(this).html())
      }
    });
  });
</script>
<div id="div">Css styled helper</div>
<ul>
  <li>One draggable element</li>
  <li>Another one</li>
  <li>And the last draggable element</li>
</ul>

If I drag any li element, the drag is not functional... but it works after the first test ! The helper div is like having a fixed position, just over the draggable li element.

The guilty file : ui.droppable.js. If I delete this file from my html header, there is no problem... but I need to use drop functionalities too ! :(

I can't revert to the latest version of the Jquery.ui package because of a lot of IE incompatibilities, which were fixed with the latest ui files.

This bug doesn't occur with IE7, Opera and Safari for Windows. I didn't try with previous versions of Firefox.

Thanks for your help and for your work about Jquery !