> If they had just changed fonts midway through a paragraph, I doubt I would have noticed beyond a simple "something's different, but I can't put my finger on it."

So you do get it, just on a subconscious level! That's exactly what good font design is all about: being invisible. It just so happens that being unnoticeable is incredibly difficult to achieve, and changes all the time with culture. A good font matches the tone of your message while remaining clear and understandable. That's because fonts aren't for robots, they're meant to be read by humans.

What Mozilla is doing here is actually a branding exercise!