Anonym isnt built into firefox, so idk why you'd think any of this has to do with other browsers.
From a privacy perspective, Anonym is only providing its customers anonymized data which has no direct reference to individual users. That's way better than say, a site using Facebook pixel being able to learn a hell of a lot about other sites you've visited and ads you've seen that are served by facebook.
Web platform security isn't about having an army of people. That's a gross oversimplification. And Mozilla already operates some massive online services that are juicy targets for hackers anyway, so it's not like they're new at this or something.