TypeError: window.clipboardData is undefined

问题：javascript实现复制到剪切板，用ff浏览器显示TypeError: window.clipboardData is undefined

<!-- lang: js -->
<script language="JavaScript"> 
function copyToBoard(id) 
{
var text=document.getElementById(id).value; 
if (window.clipboardData.setData("text",text)) 
{alert("复制成功!")} 
else 
{alert("复制失败!")} 
} 
</script>
<input id="copyId" type="text" />
<input type="button" value="复制a" onclick="copyToBoard('copyId');" />

原来clipboardData只能ie存在，在谷歌和火狐下clipboardData为null