scrollTop: offset().top not working in Firefox 4 or IE 8

scrollTop: offset().top not working in Firefox 4 or IE 8

Hello!

I have this bit of jQuery:
  1. $(document).ready(function() {
  2. $('article').click(function(e) {
  3. e.stopPropagation();
  4. var offset = $(this).offset().top - 70;
  5. $('body').animate({scrollTop: offset}, 'slow');
  6. });
It works in Chrome, but not Firefox or IE.

Here is a live example:  http://websitesthatmarket.com/dblog/site/

Thanks