Thumbnails Only Rendering in Firefox

I feel like I should know the answer to this one, but I’m stumped. I’m using the “Add Thumbnail” feature to my posts. However, in Safari, IE7, and Chrome the thumbnails aren’t appearing. And, by appearing, I mean they’re not even being output into the HTML. So, I know it’s not a filepath issue, or anything like that. Firefox is accepting the outputted HTML, and obviously showing the photos.

Any ideas what could be causing this? I’d appreciate any help. Thanks!!!

EDIT: Here’s my code:

PHP:

<?php
    $page_query = new WP_Query("showposts=1&post_type=page&page_id=21");
    while ($page_query->have_posts()) : $page_query->the_post();
    $do_not_duplicate = $post->ID;

    if ( has_post_thumbnail() ) {
     the_post_thumbnail();
    }

    endwhile;
    wp_reset_query();
  ?>

Non-Firefox Browsers:

<div class="aboutPic">
</div>

Firefox (slightly altered for anonymity):

<div class="aboutPic">
  <img width="179" height="253" src="https://---.com/wp-content/uploads/2010/03/filename.jpg" class="attachment-post-thumbnail wp-post-image" alt="" title="File name" /> </div>