原来那个是IE版滴，现在这个可以兼容FF，
可是发现FF下Select的onchange和onclick事件顺序与IE相反，
因此一旦选择就必须至少选择一项，无法全部取消，除非额外增加清空按钮！
L@_@K

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">

<html xmlns="http://www.w3.org/1999/xhtml">

<head>

    <title> new document </title>

    <meta name="generator" content="editplus" />

    <meta name="author" content="Gao YiXiang" />

    <meta name="email" content="yixianggao@126.com" />

    <meta name="keywords" content="javascript dhtml dom" />

    <meta name="description" content="I love web development." />

</head>

<body>

    <select id="sltTest" multiple size="5">

        <option value="0">-- Please select --</option>

        <option value="1">option 01</option>

        <option value="2">option 02</option>

        <option value="3">option 03</option>

        <option value="4">option 04</option>

    </select>



    <script type="text/javascript">

    <!--

Array.prototype.indexOf = function(target)

{

    var result = -1;



    for (var i=0; i<this.length; i++)

    {

        if (this[i] == target)

        {

            result = i;

            break;

        }

    }



    return result;

};



var oSlt = document.getElementById("sltTest");

oSlt.selectedList = new Array();

oSlt.onchange = function()

{

    with (this)

    {

        var i = selectedList.indexOf(selectedIndex);



        if (i == -1)

        {

            selectedList.unshift(selectedIndex);

        }

        else

        {

            options[selectedIndex].selected = false;

            selectedList.splice(i, 1);

        }



        for (var i=0; i<selectedList.length; i++)

        {

            options[selectedList[i]].selected = true;

        }

    }

};

    //-->

    </script>

</body>

</html>