Try the following:
var Comment = $('#comment_td');
if(Comment.html().trim().length == 0)
{
Comment.html('No Comments');
}
By checking the trimmed length your specifically stating to remove white space, as some browsers like IE will remove white space where as others will not, so in firefox because theres a space in the element its actually exists.
If your comments within the comments_td are in containers then you can also check the children, for example:
<td id="comment_td">
<p class="comment">A comment</p>
<p class="comment">A comment</p>
</td>
Then within jQuery you can do:
if($("#comments_td > p.comment").size() == 0)
{
//No Comments.
}