The final stage of the rollout happened a few months ago: https://blog.mozilla.org/en/mozilla/firefox-rolls-out-total-.... The last section in that article also documents the history.

(This is also relevant in any antitrust considerations for Google, as noted in the jsnell subthread here—almost all of that stuff was in early 2021, when Google was somewhere in the middle of the pack in their proposals and suggested timeline; whereas if they released it now, they’d be aligning with everyone else. Google have made some noise about delays being for the sake of advertisers at least in part, but I believe that for all the other browsers, their delays have been largely or entirely about avoiding or minimising breaking websites that depended on third-party cookies for functionality, mostly legitimately.)

So this doesn't disable 3P cookies entirely. Though it is a clever compromise.

On reviewing that article, I’m actually not certain what the situation is. It seems to suggest that it’s just isolating third-party cookies, but my understanding is that that’s roughly what they shifted to (for at least new users, and maybe that’s the key) a couple of years back. In at least Nightly, when I checked a month or two back, the default Enhanced Tracking Protection mode was Standard mode, which says that it blocks “Cross-site cookies in all windows”—not cross-site tracking cookies, but all third-party cookies.

I’m not sure why there’s so much obscuration around this stuff. I wish there was a caniuse entry on third-party cookies that identified matters clearly.