.position misbehaves in FireFox - can positions be chained?

.position misbehaves in FireFox - can positions be chained?

I'm using UI 1.8 and core 1.4.2.  I have the following script, which works in both IE and Chrome.  In FireFox, the first div, #mainNav, positions correctly, but the second div scroll nearly off the top of the page with a position (in firebug) of -768px.  In the original style, the divs both positioned absolutely, wrapped in a relatively positioned container div.  Can these two not be chained?  Do I need to do something between repositioning the first and second div, to get Firefox to recognize the new position of the first div?

  1. <script type="text/javascript">
    $(document).ready(function(){

        $("#mainNav").position({
            "my": "left top",
            "at": "left bottom",
            "of": $("#searchBar")
        });

        $("#mainContent").position({
            "my": "left top",
            "at": "left bottom",
            "of": $("#mainNav")
        })

    });
    </script>