问css在表格单元格中的绝对位置:奇怪的Firefox显示

EN

<li> 
    <div class="menu-item-container">
        <a href="#">Item</a>

        <ul>
            <li>
                <a href="#">First</a>
            </li>
            <li>
                <a href="#">Second</a>
            </li>
        </ul>
    </div>
</li>

.menu-item-container {
    position: relative;
}

<ul class="nav">
    <li>
        First level 1
        <ul>
            <li>Second level 1</li>
            <li>Second level 2</li>
        </ul>
    </li>
    <li>First level 2</li>
    <li>First level 3</li>
    <li>First level 4</li>
</ul>

ul.nav { 
    float: left; /* Needed as a kind of clearfix. Other clearfix techniques also possible */
}

/* be sure to add the > to prevent from second level li's to float */
ul.nav > li {
    float: left;
    position: relative;
}

ul.nav > li > ul {
    position: absolute;
    top: 30px;
    left: 10px;
}