This works fine in Webkit, but when I go to Firefox (Or IE8) it totally messed up. I've been staring at this for so long that I can't find any errors at all with it, so maybe one of you guys can point out where I'm going wrong.
CSS:
body {
    font-family: Georgia, serif;
    margin: 0px;
    padding: 0px;
    background: #222;
}
header {
    background: #fff url('images/header-border.gif') bottom repeat-x;
    width: 980px;
    margin: 0px auto;
    height: 100px;
    padding: 0px 0px 0px 20px;
    -moz-border-radius-topright: 4px; -moz-border-radius-topleft: 4px; 
    -webkit-border-top-right-radius: 4px; -webkit-border-top-left-radius: 4px; 
    border-top-right-radius: 4px; border-top-left-radius: 4px;
}
section {   
}
article {
}
footer {
    clear: left;
}
nav {
    width: 980px;
    margin: 0px auto;
    height: 70px;
    padding: 10px 0px 10px 0px;
    font-size: 21px;
    font-weight: bold;
    font-family: Arial, serif;
}
nav a {
    color: #fff;
    text-decoration: none;
    padding: 10px;
}
nav a:hover {
    background: #060606;
}
#content {
    width: 980px;
    background: #fff;
    padding: 0px 0px 0px 20px;
    margin: 0px auto;
}
nav ul li {
    float: left;
    list-style: none;
    width: 155px;
}
#left-column, #logo {
    width: 560px;
    margin: 0px auto;
    float: left;
}
#right-column, #share {
    width: 380px;
    margin: 0px auto;
    float: left;
    height: 100%;
    padding: 0px 0px 0px 20px;
    border-left: 1px solid #d9d9d9;
}
address {
    display: inline;
}
a img {
    border: 0px;
}
.clear-left {
    clear: left;
}
And the HTML:
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="text/html; charset=UTF-8" />
<title>Webtint </title>
<link rel="stylesheet" href="http://localhost/wp-content/themes/clean/style.css" type="text/css" />
<link rel="pingback" href="http://localhost/xmlrpc.php" />
</head>
<body>
<nav>
    <ul>
        <li><a href="#">home</a></li>
        <li><a href="#">tutorials <img src="http://localhost/wp-content/themes/clean/images/arrow.gif" alt="Arrow" /></a></li>
        <li><a href="#">resources <img src="http://localhost/wp-content/themes/clean/images/arrow.gif" alt="Arrow" /></a></li>
        <li><a href="#">articles <img src="http://localhost/wp-content/themes/clean/images/arrow.gif" alt="Arrow" /></a></li>
        <li><a href="#">contact</a></li>
        <li><a href="#">follow us</a></li> 
    </ul>
</nav>
<header>
    <section id="logo">
         
    </section>
    <section id="share">
         
    </section>
</header>
<div id="content">
    <section id="left-column">  
        <article>
            <h2>
                <a href="<?php the_permalink() ?>" rel="bookmark" title="Permanent Link to <?php the_title_attribute(); ?>">
                        <?php the_title(); ?>
                </a>
            </h2>
            <time><?php the_time('F jS, Y') ?></time> by <address><?php the_author(); ?></address>
            <?php the_excerpt(); ?>
            <br /><br />
        <a href="<?php the_permalink(); ?>">Read More</a> 
        <?php comments_popup_link(
        '<span class="boxed">No Responses »</span>', 
        '<span class="boxed"> 1 Response »</span>',
        '<span class="boxed">% Responses »</span>'); ?>
    </article>
    <hr />
    </section>
    <section id="right-column">
             <h2>Popular Posts</h2>
    </section>  
</div>
<footer>
</footer>
</body>
</html>
Screenshot in Firefox: http://imgur.com/wvhI0.gif
Screenshot in Chrome: (How it's supposed to look) http://imgur.com/sQK8S.gif
Screenshot in IE8: http://imgur.com/7OnEJ.gif
Thanks for any help in advance :)