firefox 下取frameset对象与ie不兼容问题

document.getElementById("mainArea").contentWindow.document.getElementById("mainFrameset");

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主页面中包含? <iframe scrolling="no" src="indexFrame.jsp" id="mainArea" name="mainArea" width="100%" height="100%" frameborder="0" marginwidth="0" marginheight="0" ></iframe>

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子页面中包含

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<frameset rows="135,*,44" cols="*" frameborder="no"? framespacing="0" border="0" name="mainall">
<frame src="${ctx}/manageIndex.do?method=top" name="topFrame" scrolling="NO" noresize marginwidth="0" marginheight="0"/>
<!--frame src="${ctx}/manageIndex.do?method=index" name="mainFrame" scrolling="yes" noresize marginwidth="0" marginheight="0"/-->
<frameset id="mainFrameset" name="mainFrameset" cols="0,*" frameborder="no" border="1" framespacing="0" >
??? <frame id="leftFrame" src="" name="leftFrame" scrolling="yes" noresize marginwidth="0" marginheight="0"/>
????? <frame src="${ctx}/manageIndex.do?method=index" name="main" scrolling="YES" noresize marginwidth="0"/>?
? </frameset>
<frame src="${ctx}/bottom.jsp" name="bottomFrame" scrolling="NO" noresize marginwidth="0" marginheight="0"/>
</frameset>

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在主页面中调用子页面中的frame对象可以如上操作