I thought Firefox enabled this automatically providing your graphics card drivers are up to date. If I force it enabled on my old GeForce Go 7600 all the text comes out as black rectangles. I assumed Mozilla knew about these problems so only enabled it when they knew it'd be reliable. So maybe leave it to Mozilla?
<a href="http://support.microsoft.com/kb/2545698" rel="nofollow">http://support.microsoft.com/kb/2545698</a><p>this supposedly fixes how directwrite works in ie9, but since it updates commonly used fonts themselves, I assume it could help the way firefox renders too.