为什么这一段在firefox里不弹出alert?
为什么这一段在firefox里不弹出alert? 
  <HTML>  
  <SCRIPT>  
 function   fnBegin(){ 
 var   fr   =   document.getElementById( "oFrame ").cloneNode(); 
 alert(document.body.innerHTML); 
 } 
  </SCRIPT>  
  <BODY   onload= "fnBegin() ">  
  <IFRAME   id= "oFrame "   src= "http://www.163.com " 
 style= "border:1px   solid   black;   position:absolute;   top:20px;   left:30px; 
 width:350px;   height:300px; ">  </IFRAME>  
  </BODY>  
  </HTML>
------解决方案--------------------var fr = document.getElementById( "oFrame ").cloneNode(true);