Comment 20 for bug 589485

(In reply to comment #19)
> 96 is almost always the wrong
> choice. The number seems to have come out of nowhere and makes little
> sense as a default.

It was chosen in order to make display of web pages using Xorg more consistent with the way they get displayed on Windows, which by default assumes 96. http://blogs.msdn.com/fontblog/archive/2005/11/08/490490.aspx explains the origin of 96.