A minor layout corruption may occur in FF running on Win XP during slider operation. Two small areas will be marked by the browser as selected. It seems that it comes from the two divs:
<div class="webix_slider_left" style="width: 119px;"> </div>
<div id="xsld" class="webix_slider_handle" style="left: 121px;"> </div>
Removing the div content (innerHTML) cleans also the layout.