Firefox issues with boxes and negative margins

I’m working on a new <a href=”http://dynamic.gamespy.com/~populous/forum/”>forum</a> design and it’s going pretty nicely except there’s a nasty bug in firefox.
Relevant CSS:

[code=php]
.post {
padding-left:10.5em;
min-height:16em;
}
.author {
width:10em;
background-color:#003;
border-right:solid 2px #191970;
margin-left:-10.5em !important;
margin-left:-5em;
}
.clear {
clear:both;
}
[/code]

Relevant XHTML:

[code=php]
<div class=”post”>
<div class=”author”>
author info
</div>
rest of post
</div>
<hr class=”clear” />
(repeats)
[/code]

Basically the author box shows up next to the post info as it should, and in Opera and IE the <hr /> shows up below the author box when it extends past the post box (when the post box is small and the author box is big, it will just extend out of the post box). In firefox on the otherhand, it just ignores the clear and continues into the box below overlapping. Is there some special way to prevent this?

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