We need more information than that, as we certainly would have noticed behaviour like that were it widespread. (FWIW, I can't reproduce the problem, so...)
Are you using the WYSIWYG editor or standard textarea?
Can you reproduce the bug here, or does it only happen on your own board?
Are you using Windows Firefox, Mac Firefox or Linux Firefox?
This is an issue in TinyMCE itself - you can see it happen in their own demos. I think it might actually be an underlying FF issue, as it seems to be highlighting the <p>, even if there's no content in it.