tab标签，在IE7下正常，在火狐4下就不正常

<div class="tab_menu">
        <ul>
            <li class="selected">工程</li>
            <li>地名</li>
            <li>经纬度</li>
        </ul>
    </div>

 .tab { width:716px;text-align: left; margin:0px auto 0px auto;}
 .tab_menu { clear:both; margin:0px; height:20px;}
 .tab_menu li { float:left; text-align:center; cursor:pointer; list-style:none; padding:1px 6px; margin-right:4px; background:#F1F1F1; border:1px solid #898989; border-top:none;}
 .tab_menu li.hover { background:#DFDFDF;}
 .tab_menu li.selected { color:#FFF; background:#6D84B4;}
 .tab_box { clear:both; border:1px solid #898989; height:100px;}