I've been working on blocking out a page layout, and after I upgraded to Firefox 4 I noticed that a central element containing 3 floating elements and a clearing div suddenly started gaining spacing where the top margin would be whenever the bottom margin was set. One can bypass the problem by either giving the clearing div a height somehow or by using padding instead, but I want to know if this is a bug or that I'm just missing something. This behavior is not duplicated in any other browser I've tried including IE6. I have set up a demo at http://jacobanderson.us/projects/bugs/firefox4margintest.html - if you edit the bottom margin of the "body .page .content .keynote" element in Firefox 4 you'll see that the entire element gains spacing on both the bottom and top equally. The same effect can be triggered by adding a top margin to the "body .page .content .columns" element.