In today's Nightly (2013-09-21), XUL Button elements have developed a curious quirk. They take their parent element's background color, but not foreground color, causing visibility issues. In yesterday's nightly, they followed OS UI standards when placed in an HTML element, today they seem to follow that HTML element, partially. I discovered the problem when using a darkened version of an edit of the NetError page, available from <http://realityripple.com/Software/Themes/In_The_Dark/Mozilla/fierr_in_the_dark.xpi>. If you pull out your inspector and edit the errorPageContainer div's background color, you'll notice that the un-formatted button objects within it follow. However, changing the foreground color does nothing. The buttons should follow OS standards for buttonface.
It's gotten way worse today... Way worse. The background is now an entire an element down and to the right, completely in the wrong place.
After a bit more experimentation, it seems to be directly related to the Opacity of the parent object. Looks like XUL buttons hate HTML transparency.
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