onclick = function (e)的疑惑

<%@ Page Language="C#" AutoEventWireup="true" CodeFile="__mouseMoveDrop.aspx.cs" Inherits="__mouseMoveDrop" %>

<!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 runat="server">
    <title></title>

</head>
<body>
    <form id="form1" runat="server">
    <div id="tt" style=" background-color:Gray;height: 59px;">

    </div>
    </form>
        <script type="text/javascript">
            document.getElementById('tt').onclick = function () {
                alert('eeeee');
            }

    </script>
</body>
</html>

其实在firefox底下是有onclick 事件的