Why is Firefox not properly rendering this textarea element?

I made some changes to one of my web pages (CSS and HTML) and now Firefox is not rendering this textarea properly. It has a sort of jitter effect happening in the middle开发者_运维问答 of the top line. I'm not sure if it has anything to do with the changes I made, but what could be causing this?

<div id="CommentForm">
    <form action="/posts/add-comment" method="post">
        <input type="hidden" id="Table_Name" name="Table_Name" value="Posts" />
        <input type="hidden" id="Table_ID" name="Table_ID" value="47" />
        <input type="hidden" id="IsLiveBroadcast" name="IsLiveBroadcast" value="1" />
        <textarea name="Comment_Body" id="Comment_Body" rows="1"></textarea>
        <div id="error-messages"></div>
        <input type="submit" class="floatRight" id="postComment" value="Post your response" />
    </form>
    <div class="clear"></div>
</div>

And the CSS being applied:

#live_broadcast_post #CommentForm {
    margin: 0;
    padding: 0;
    width: 460px;
    height: auto;
    background: none;
}

#live_broadcast_post #CommentForm textarea {
    font-family: "Lucida Grande",Verdana,"Bitstream Vera Sans",Arial,sans-serif;
    font-size: 12px;
    padding: 5px;
    width: 98%;
}

The strange thing is that this wasn't happening before I made the changes, and now it only affects Firefox, not any other browser I have tested.

Update:

I noticed that when the page loads, it renders correctly. Only after an iframe (also on this page) is finished loading does the rendering change.

Seeing as even the text itself is affected, this looks rather like a OS / graphics card / rendering problem.

I would try it on a different machine first before questioning the markup.

I've been seeing this jitter in some websites, not with Firefox only. Probably graphics rendering (cards) related.