Firefox currently renders its own fonts. If you have the epiphany browser installed, you can see the difference of their rendering and you might even notice that epiphany renders fonts better at some extent.
If you want Firefox to use ubuntu's own freetype font rendering engine, you can do this:
1. Go to an internal website in Firefox called about:config.
2. Find font.Freetype2.autohinted, font.Freetype2.enable and set them as true.
3. Find font.Freetype2.unhinted and set it as false.
You're done. If you like the old Firefox rendering though, you can just do the opposite of what we just did.
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