Add font smoothing for Firefox

When viewing pages in Firefox OSX, the fonts appear heavier than
specified.
Using -moz-osx-font-smoothing: grayscale; fixes this, more information
on this here:
https://bugzilla.mozilla.org/show_bug.cgi?id=857142
Merge pull request #106 from gemmaleigh/fix-chunky-fonts-in-firefox

Add font smoothing for Firefox