js获取当前浏览器版本

<script type="text/javascript">
var Sys = {};
var ua = navigator.userAgent.toLowerCase();
if (window.ActiveXObject)
Sys.ie = ua.match(/msie ([\d.]+)/)[1]
else if (document.getBoxObjectFor)
Sys.firefox = ua.match(/firefox\/([\d.]+)/)[1]

if(Sys.ie) document.write('IE: '+Sys.ie);
if(Sys.firefox) document.write('Firefox: '+Sys.firefox);
</script>