var userAgent = navigator.userAgent.toLowerCase();

if(userAgent.match(/firefox/)) {
    $( '#'+elm_id ).bind( "sortstart", function (event, ui) {
        ui.helper.css('margin-top', $(window).scrollTop() );
    });
    $( '#'+elm_id ).bind( "sortbeforestop", function (event, ui) {
        ui.helper.css('margin-top', 0 );
    });
}