According to (https://www.dz-techs.com/en/first-party-isolation-firefox), "In short, First-Party Isolation (FPI) limits access to cookies, cache, and similar data to the domain level only. This means that when FPI is enabled, advertising agencies cannot use cookies to track your online activity while browsing the web." FF introduced this option not long ago.
Of course I read a lot more than this and in my guesstimation this is for people who would rather put their cookies in containers, rather than use Containers.
I've been giving it a go....the sad part is there is no way possible to know this works unless you have forensics kung fu.
Omitted: the 2 or so about:config moves you have to make for this. Easy to find.
