Edit in JSFiddle

<nav>
    <a id="menu-toggle" href="javascript:void(0);">&#9776;</a>
    <ul>
        <li><a href="http://mozilla.com.tw/firefox/">Firefox</a></li>
        <li><a href="http://mozilla.com.tw/firefox/os/">Firefox OS</a></li>
        <li><a href="http://mozilla.com.tw/community/">社群參與</a></li>
        <li><a href="http://mozilla.com.tw/news/">訊息中心</a></li>
        <li><a href="http://mozilla.com.tw/about/">參與我們</a></li>
    </ul>
</nav>
<article>
    <p>&nbsp;</p>
    <p>&nbsp;</p>
    <p>&nbsp;</p>
    <p>我是內文</p>
    <p>&nbsp;</p>
    <p>&nbsp;</p>
    <p>&nbsp;</p>
</article>

body {
    text-align: center;
}

a {
    color: #888888;
    text-decoration: none;
}

nav {
    display: inline-block;
    vertical-align: top;
    text-align: left;
}
nav ul {
    margin: 0;
    padding: 0;
    background: #CCFFCC;
}
nav ul li {
    list-style-type: none;
    padding: 6px 12px;
    border-bottom: 1px solid #FFF;
}
nav ul li:hover {
    background: #88CC88;
}
nav ul li:hover a {
    color: #FFF;
}
article {
    display: inline-block;
    width: 280px;
    background: #FFFFCC;
    vertical-align: top;
}

#menu-toggle {
    position: fixed;
    width: 30px;
    height: 30px;
    display: none;
    background: #CCFFCC;
    text-align: center;
    line-height: 30px;
    vertical-align: middle;
}

@media only screen and (max-width: 400px) {
    nav {
        position: fixed;
    }
    nav > ul {
        display: none;
    }
    #menu-toggle {
        display: block;
    }
    nav:hover > ul {
        display: block;
        margin-top: 30px;
    }
}