it's a bug in intel OSX.
apps using quickdraw (mostly carbon apps) do not respect the system-wide font smoothing settings.
made a bug report #4598078 in ADC bug reporter
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On Intel Macs, the system-wide font anti-aliasing setting is ignored on apps that use QuickDraw to draw fonts (mostly Carbon apps), such as Firefox, iTunes, Word etc.
So regardless of the font anti-aliasing setting you have, the ugly blurry greyscale font anti-aliasing is done instead of the subpixel rendering.
This issue is causing a major strain to my eyes.
Steps to Reproduce:
Choose Subpixel rendering in Font preferences.
Open Apps that uses QuickDraw such as iTunes or Firefox.
Using apps like Digital Colormeter, look at the fonts and see that there is no colored fringes, i.e. no subpixel rendering.
Actual Results:
No subpixel rendering is done on apps that use QuickDraw (mostly Carbon apps).
Regression:
This only occurs on Intel-based Macs.