求各位大大，一段JS代码修改，求在火狐下能实现拖动层

// JavaScript Document

var md=false,mobj,ox,oy;
function document.onmousedown(){
   if(event.srcElement.forid=="area")
   {
    md = true;
    mobj = document.getElementById(event.srcElement.id);
    ox = mobj.offsetLeft - event.x;
    oy = mobj.offsetTop - event.y;
   }
}
document.onmouseup=function(){md=false;}
function document.onmousemove()
{
   
   if(md)
   {
    var top = event.y + oy;
    window.status = top;
    if(!(top < 0 || top > 195)){
     mobj.style.top = top;
     var step = ((document.getElementById('con').clientHeight*2 - 195) / 500);
     document.getElementById('con').style.marginTop = ((0 - top) * step);
    }
   }
}

在IE下能实现拖动层了，就是火狐下没动静