In Firefox where do I find the setting: "Use LCD font smoothing when available"?
That in not a setting in Firefox, but in the Windows display (appearance) settings.
If ClearType is enabled in Windows (and this is the default in Windows 7) then Firefox uses it automatically.
- Control Panel > Personalization > Window Color and Appearance > Fonts > "Adjust ClearType text"
- Anti-Aliasing Tuner: https://addons.mozilla.org/firefox/addon/anti-aliasing-tuner/
If there is a problem then try to disable hardware acceleration in Firefox.
- Tools > Options > Advanced > General > Browsing: "Use hardware acceleration when available"
You can also try to set the gfx.content.azure.enabled pref to false or if this didn't help disable Direct2D by setting the gfx.direct2d.disabled pref to true on the about:config page and leave hardware acceleration otherwise enabled.