jQuery UI Sortable: Firefox :active issue

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

HTML

<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

$('ul.sortable').sortable();