[jQuery] FF3 +jquery 1.2.6 + thickbox + custom plugin issue

[jQuery] FF3 +jquery 1.2.6 + thickbox + custom plugin issue

Hello!
I have a strange issue, since Firefox 3 came up: i use thickbox to launch a page inside a modal window. On this page runs a custom plugin that resizes automatically a div to the available window height. I then use jscrollpane to have a good looking custom scrolbar appended to that div.
What happens now is that on page load the new height calculation amounts to 0. If i refresh the page, then the height is correctly evaluated.
Here is the plugin i made, can someone help me find and correct the bug? I've tried everything i could to make it work, but so far... failed. The bug only occurs with Firefox 3.
jQuery.fn.setScrollableArea = function(modifier){
    var availableHeight = jQuery(window).height();
    return this.each(function(){
        var offset = jQuery(this).offset();
       
        modifier = (typeof modifier == 'undefined') ? 0 : modifier;
        var newHeight = availableHeight - offset.top - modifier;
        alert('available height='+availableHeight+ "\n top="+offset.top); // all values are calculated as 0 until i refresh the page !!
        jQuery(this).css({
            height: newHeight,
            overflow: 'hidden'
        });
    });
}
I call it everytime i need it via:
$(document).ready(function(){
    $('#section_content').setScrollableArea(30).jScrollPane({
        scrollbarWidth: 13,
        scrollbarMargin: 10,
        animateTo: true
    });
});
Any help would be much appreciated.
Thank you,
Alexandre