So for some unknown reason suddenly this morning all my fonts looked like garbage in Firefox and I saw no font smoothing or other options in the default settings page. I did find a tweak though that fixed it.
In a new tab, type or paste, “about:config” in the address bar and press Enter/Return. Click the button saying you know you can break shit here.
Search for, “font_r” in the search bar.
Look for, “gfx.font_rendering.cleartype_params.rendering_mode” and click the pencil / edit button on the right and change the -1 to a 2.
Click the pencil icon / check mark to save the change.
Restart Firefox and that should make your font;s not look like crap.