为何火狐中我的CLIP属性不起作用?

现有一段CSS,其CLIP元素在IE中可以剪裁,但在火狐中却无效,请高人指导:
#before
{
position:absolute;
opacity:1;
-moz-opacity:1;
-khtml-opacity:1;
filter:alpha(opacity=100);
top:200px;
clip:rect(50%,auto,auto,auto);

}