代码不能在firefox中正常运行.请大虾帮改

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=gb2312" />
<title>无标题文档</title>
<script type="text/javascript" language="javascript">
function show(evt){
document.getElementById("ui").style.top=evt.offsetY || evt.layerY + "px";
document.getElementById("ui").style.left=evt.offsetX || evt.layerX + "px";
document.getElementById("ui").style.display="";
}
</script>
</head>
<body>
<div id="ui" style="background:#FF3300; position:absolute; height:100px; width:100px; display:none;"></div>
<p><span onclick="show(event)">1111111111</span><br />
</p>
<p>&nbsp;</p>
<p>&nbsp;</p>
<p>&nbsp;</p>
<p>&nbsp;</p>
<p>     <span onclick="show(event)">1111111111</span><br />
</p>
</body>
</html>