Mozilla says they use a third-party OHTTP intermediary. In the blog post linked above, they name Fastly as their partner. So it's not as bad as Mozilla + Mozilla-wearing-funny-glasses.
Personally, I still think this is the wrong approach to privacy, even though I've used Fakespot on my own many times over the years. Largely because I don't think any of this needs to be built into a web browser.
I would prefer my web browser to minimize information leakage by default, to the greatest degree that it can while still remaining useful as a web browser. Mozilla keeps adding bloat to Firefox, and bloat always comes at a cost. I'd much prefer these to be browser extensions that people can download if they want them, rather than built in by default. The baseline Firefox should be lean. Less "stuff" = smaller attack surface. Simplicity is best.
I mean, the Fakespot browser extension has existed for a long time, and I've never seriously considered installing it. I'd much rather just take an extra three seconds to load their web site and paste in a URL than have it constantly monitoring my activity and doing god-knows-what with it. That way I have better knowledge and control of what is happening with my data. Even if I trust their intentions, I don't implicitly trust their competence (all software has bugs) and I don't trust that they will never go rogue in the future.
And also, I just don't find this claim all that compelling in principle:
By processing the data jointly across two independent parties, they ensure neither party holds the information required to reveal sensitive information about someone.
I mean...sure. That's fair. Buuuuuut handing half the data to your "partner" doesn't give me a whole lot of confidence. Especially since literally nobody reads all of the privacy policies they are subject to. See:
https://www.theatlantic.com/technology/archive/2012/03/reading-the-privacy-policies-you-encounter-in-a-year-would-take-76-work-days/253851/
https://www.npr.org/sections/alltechconsidered/2012/04/19/150905465/to-read-all-those-web-privacy-policies-just-take-a-month-off-work
https://www.techradar.com/computing/cyber-security/you-need-a-whole-workweek-every-month-to-read-privacy-policiesand-thats-bad-news
Minimizing privacy policies should be a high-priority goal for any organization that claims to value privacy.
Furthermore, how many additional parties have access (legally or otherwise) to both Mozilla and Fastly? 🤷