Added support for firefox osx font smoothing.

Related to issue #1591
Based on https://bugzilla.mozilla.org/show_bug.cgi?id=857142#c83
Signed-off-by: Martin Seeler <developer@chasmo.de>
Merge pull request #1727 from MartinSeeler/4.0.0-wip

Added support for font smoothing in Firefox 25 on Mac OS X