In GNOME, we need to use a cairo package that make freetype2 subpixel working with it, I found one in fedora forum works with me, thanks to Anshul.
/dev/loki: Freetype2 packages with subpixel hinting
But it still not working with firefox 3
I think we need to build firefox with --enable-system-cairo
Subpixel font hinting in Firefox 3.0 b3 - Ubuntu Forums
Could someone try to do it please?