Annoying bug using Firefox on a Mac. If you have a hidden div and it has scrollbars, the scrollbars will shine through:
https://bugzilla.mozilla.org/show_bug.cgi?id=187435
The simple fix, using overflow: hidden when the div is not being displayed doesn’t work for a web site I’m building, www.mbnnyc.com, which has a hidden div that is revealed by animating the div. Changing the order of the divs works partially, although some of the scrollbar still shines through.