Me, and I guess most other programmers have their own favourite monospace
font.  And they most probably have set up their browsers to use that font as
the "monospace" generic font family.

But GitHub, a site for programmers, of all places, just plain ignores this
preference.  What's worse, they use "Courier New" (sometimes just "Courier")
as a way to make my eyes hurt every time I want to read code on the site.

I have nothing against "Courier New", except for the following:

 - It is not very well suited for code listings (due to some well known almost
   indistinguishable glyphs), and

 - The font rendering is quite thin on my operating system, which, combined
   with light colors no not-really-white background makes my eyes hurt when
   trying to read the code.

According to [1] GitHub has decided not to do the right thing and make the
programmers all around the world suffer.

This extension only provides a stylesheet that overrides all selectors that
mention "Courier" by setting "font-family" to "monospace".  I did this with a
quick Emacs keyboard macro so it may well not be perfect, and will probably
get out of date at some near point in future.  But I totally refuse to invest
any more time on coming up with a more reliable and "proper" solution when the
right thing(TM) is for GitHub to do their homework.

Footnotes: 
[1]  http://support.github.com/discussions/site/937-please-dont-use-specific-monospace-fonts