FireFox 下 JavaScript求助，请高人指点在线等待,急。

图片地址你换下
<body>
<STYLE type=text/css>
.sec1 {
BORDER-RIGHT: gray 0px solid; BORDER-TOP: #ffffff 1px solid; BORDER-LEFT: #ffffff 0px solid; COLOR: #ffffff; BORDER-BOTTOM: #ffffff 0px solid; BACKGROUND-COLOR: #ffffff
}
.sec2 {
BORDER-RIGHT: gray 0px solid; BORDER-TOP: #ffffff 0px solid; FONT-WEIGHT: bold; BORDER-LEFT: #ffffff 0px solid;COLOR: #ffffff; BACKGROUND-COLOR: #ffffff
}
.main_tab {
BORDER-RIGHT: gray 0px solid; BORDER-LEFT: #ffffff 0px solid; COLOR: #ffffff; BORDER-BOTTOM: gray 0px solid; BACKGROUND-COLOR: #ffffff
}
</STYLE>
<!--JavaScript部分-->
            <SCRIPT language=javascript>
     function secBoard(n)
  {  var secRow=document.getElementById("secTable").rows[0];
    for(i=0;i<secRow.cells.length;i++)
      secRow.cells[i].className="sec1";
    secRow.cells[n].className="sec2";
    var objdiv=document.getElementById((n==0)?"div1":"div2")
    document.getElementById("div1").style.display="none";
    document.getElementById("div2").style.display="none";
    objdiv.style.display="block";
    
  }
      </SCRIPT>
  
<table width="400" height="" border="0" align="center" 
            cellpadding="0" cellspacing="0" class="main_tab" id="mainTable">
                        <!--关于TBODY标记-->
                          <tr >
                            <td >
                             <div id="div1" width="400" height="69" align="middle" style="DISPLAY: block">
                              <table width="400" border="0" align="center" cellpadding="0" cellspacing="0">
                                <tbody>
                                  <tr>
                                    <td height="29"><img src="http://community.csdn.net/logo/images/prj.210.67.gif" width="400" height="400" /></td>
                                  </tr>
                                </tbody>
                            </table>
                          </div>
                            </td>
                          </tr>                          <tr>
                            <td>
                             <div id="div2" width="400" height="69" align="middle" style="DISPLAY: none">
                              <table width="400" border="0" align="center" cellpadding="0" cellspacing="0">
                                <tbody>
                                  <tr>
                                    <td height="400"><img src="http://zi.csdn.net/ut300_60.gif" width="400" height="400" /></td>
                                  </tr>
                                </tbody>
                            </table>
                          </div>
                            </td>
                          </tr>
                        </table>
                        <br />
                        <table width="411" border="0" align="center" cellpadding="10" cellspacing="0" id="secTable">
                          <tbody>
                            <tr align="middle" height="20">
                              <td width="137" height="85" class="sec2" onclick="secBoard(0)"><div align="center"><img src="http://zi.csdn.net/ut300_60.gif" width="137" height="85" /></div></td>
                              <td width="137" height="85" class="sec1" onclick="secBoard(1)"><div align="center"><img src="http://zi.csdn.net/ut300_60.gif" width="137" height="85" /></div></td>
                              </tr>
                          </tbody>
                      </table>
</body>