I want to disable ClearType fonts rendering from all programs, including Firefox and any other random program I might be using that uses this feature.
This does not help me (in fact its already set to standard for me):
OS: Windows XP SP3
Have you attempted to Tune the ClearType system-wide?
Firefox attempts to handle it's own font smoothing, so perhaps check out the FF "Anti-Aliasing Tuner" plug-in, and/or type turning off Hardware Acceleration in FF, so that it stops trying to smooth the fonts. :)