jQuery UI Sortable: Firefox :active issue (jQuery 1.6.4)

Pseudo class :active gets suppressed by ui.sortable in Firefox

HTML

<link rel="stylesheet" href="http://ajax.googleapis.com/ajax/libs/jqueryui/1.8.16/themes/base/jquery-ui.css">
<link rel="stylesheet" href="http://static.jquery.com/ui/css/demo-docs-theme/ui.theme.css">
<script src="http://ajax.googleapis.com/ajax/libs/jqueryui/1.8.16/jquery-ui.min.js"></script>
<script src="http://jquery-ui.googlecode.com/svn/tags/latest/external/jquery.bgiframe-2.1.2.js"></script>
<script src="http://ajax.googleapis.com/ajax/libs/jqueryui/1.8.16/i18n/jquery-ui-i18n.min.js"></script>
<ul class="sortable">
    <li>Item 1</li>
    <li>Item 2</li>
    <li>Item 3</li>
    <li>Item 4</li>
</ul>

CSS

.sortable {
    list-style: none;
    padding: 0;
}
.sortable li {
    padding: 0.5em 1em;
    margin: 1em;
    border: thin solid black;
    cursor: pointer;
}
.sortable li:active {
    background: orange;
    cursor: move;
}

JavaScript

$(".sortable").sortable();