> My understanding is that Firefox has supported in-browser blocking of third-party ads, trackers, fingerprinters, &c., for a while now

I see the part about trackers. What about ads? Does it also block YouTube ads? That's a really nice feature built into Brave.

> just about every adtech company that promises "privacy preserving targeted advertising" via ML

The difference here is it happens in the browser. Does ML happen in Chrome?

> Brave's own site implies that local ad ML is currently opt-in only

Your reading of that isn't correct. The opt in is for ads. And local machine learning comes along with opting in.