Problem: the [ code ] tag gets converted to:
<blockquote><pre><font face="verdana,arial,helvetica" size="1">code:</font><hr>Inserted text<hr></pre></blockquote>
Unfortunately this causes
Bug 60992 to occur; the horizontal rules above and below the code get shortened to just 1 pixel long. This is by no means your fault, and neither is it Mozilla's - this is due to Mozilla trying to replicate the behaviour of earlier browsers, and this is an unfortunate side-effect.
Could you consider moving the <hr>s outside of the <pre> element so it displays properly in Mozilla?
Many thanks!