请教一个火狐下的透明度问题.

var _showder_div,_bigschool_msssageBox_main,_bigschool_msssageBox_subProvinces,_bigschool_msssageBox_mainContent,_page_width,_page_height;
function showShowder()
{
    _page_height = document.body.scrollHeight;
    _page_width = document.body.scrollWidth;
    
    _showder_div = document.createElement("div");
    _showder_div.setAttribute("id","ajax_div");
    _showder_div.style.width = _page_width + "px";
    _showder_div.style.height = _page_height + "px";
    _showder_div.style.zIndex = 100;
    _showder_div.style.MozOpacity = 0.5; //FF设置这里
    _showder_div.style.background = "#999999";
    _showder_div.style.top = "0px";
    _showder_div.style.left = "0px";
    _showder_div.style.position = "absolute";
    _showder_div.style.filter = "alpha(opacity=20);-moz-opacity:0.2";
    document.body.appendChild(_showder_div);
}