function myBrowser(){
var userAgent = navigator.userAgent; //取得瀏覽器的userAgent字符串
var isOpera = userAgent.indexOf("Opera") > -1; //判斷是否Opera瀏覽器
var isIE = userAgent.indexOf("compatible") > -1 && userAgent.indexOf("MSIE") > -1 && !isOpera ; //判斷是否IE瀏覽器
var isFF = userAgent.indexOf("Firefox") > -1 ; //判斷是否Firefox瀏覽器
var isSafari = userAgent.indexOf("Safari") > -1 ; //判斷是否Safari瀏覽器

if(isIE){
   var IE5 = IE55 = IE6 = IE7 = IE8 = false;
   var reIE = new RegExp("MSIE (\\d+\\.\\d+);");
   reIE.test(userAgent);
   var fIEVersion = parseFloat(RegExp["$1"]);

   IE55 = fIEVersion == 5.5 ;
   IE6 = fIEVersion == 6.0 ;
   IE7 = fIEVersion == 7.0 ;
   IE8 = fIEVersion == 8.0 ;

   if(IE55){ return "IE55"; }
   if(IE6){ return "IE6"; }
   if(IE7){ return "IE7"; }
   if(IE8){ return "IE8"; }
}//isIE end

if(isFF){ return "FF"; }
if(isOpera){ return "Opera"; }

}//myBrowser() end

//以下是調用上面的函數

if(myBrowser()=="FF"){alert("我是 Firefox");}
if(myBrowser()=="Opera"){alert("我是 Opera");}
if(myBrowser()=="Safari"){alert("我是 Safari");}

if(myBrowser()=="IE55"){alert("我是 IE5.5");}
if(myBrowser()=="IE6"){alert("我是 IE6");}
if(myBrowser()=="IE7"){alert("我是 IE7");}
if(myBrowser()=="IE8"){alert("我是 IE8");}