禁用Tab键JS代码兼容Firefox和IE

<script type="text/javascript"> 
document.onkeydown=function(e){ 
var isie = (document.all) ? true:false; 
var key; 
var ev; 
if(isie){//IE浏览器 
key = window.event.keyCode; 
ev = window.event; 
}else{//火狐浏览器 
key = e.which; 
ev = e; 
} 
alert(key); 
if(key==9){//IE浏览器 
if(isie){ 
ev.keyCode=0; 
ev.returnValue=false; 
}else{//火狐浏览器 
ev.which=0; 
ev.preventDefault(); 
} 
} 
}; </script>