This page has the #content (<div> with float: left) and the #sidebar (<div> with float:right) contained in a #page <div> with the overflow property set to 'auto' and left and right padding of 20px..
You don't see the float problem here at first, because there is not enough text in the #sidebar to fill the screen vertically. However if you resize your browser window or if you switch styles and come back to this style (the 'Normal' style) you see the problem occur.
What you do see is that Firefox adds whitespace for no apparent reason. Well there is at least some sort of explanation : the 'white space' under the #content is the same height as the #sidebar...