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?
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.
