I thought the article made it pretty clear that the heuristics are only intended as a temporary stop-gap measure to prevent every current SSO service from breaking.
That sentence alone is mind boggling - Hey, Mozilla is breaking every standard that previously allowed cross domain authentication, or 3rd party authentication services, but don't worry, it's only temporary... until what exactly?
Either every site under the sun is suddenly asking the user to accept unpartitioning because critical features are broken and a user literally has to click yes to continue, in which case all you're doing is training users to always click yes (EU cookie popups, take two, lovely...)
Or users move away from Firefox because sites are just broken.
They don't even have a long term solution for how SSO services are expected to work here, other than "use this other API that happens to cause the behavior as a side effect, and oh, btw, the global market leader (chrome) doesn't support it yet! Enjoy!"
Where the fuck is my RFC. Where is real planning I'd expect from a from the company that I only know the name of because they actually gave a shit about standards when IE was doing this kind of crap left and right?
I like Firefox precisely because they normally don't do this bullshit (and their standards based docs on MDN are literally some of the best around).
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My take is that this is essentially security advertising and social signaling, and is another death blow to Firefox from Mozilla trying to push privacy focused paid services.