<script language=javascript>
function secBoard(n)
{
var menu=document.getElementById("menu");
var main=document.getElementById("main");
for(i=0;i<menu.childNodes.length;i++)
menu.childNodes[i].className="sec1";
menu.childNodes[n].className="sec2";
for(i=0;i<main.childNodes.length;i++)
main.childNodes[i].style.display="none";
main.childNodes[n].style.display="block";
}
</script>
<div id="header">
<ul id="menu">
<li onMouseOver="secBoard(0)" class="sec2">一号</li>
<li onMouseOver="secBoard(1)" class="sec1">二号</li>
<li onMouseOver="secBoard(2)" class="sec1">三号</li>
<li onMouseOver="secBoard(3)" class="sec1">四号</li>
</ul>
<ul id="main">
<li class="block">一号内容</li>
<li class="unblock">二号内容</li>
<li class="unblock">三号内容</li>
<li class="unblock">四号内容</li>
</ul>
</div>

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因为firefox中childNodes会获取textNode所以你这些编号就不对了 最好用getElementsByTagName('li')来获取元素就没问题了


function secBoard(n)
{
var menu=document.getElementById("menu");
var main=document.getElementById("main");
var menuli=menu.getElementsByTagName("li");
for(i=0;i<menuli.length;i++)
	menuli[i].className="sec1";
	menuli[n].className="sec2";
var mainli=main.getElementsByTagName("li");
for(i=0;i<mainli.length;i++)
	mainli[i].style.display="none";
	mainli[n].style.display="block";
}