First party isolation was added in FF55 https://www.bleepingcomputer.com/news/software/another-tor-b...
There's a First Party Isolation add-on though if you want a reachable toggle button.
Exception: gitea uses Github oauth
Exception: unbury.me uses Google login
etc
Default: Full FPI
It would be neat to see Mozilla create an intuitive UI for this to enable first party isolation by default, like a permissions model for other things. A modal popup "Unbury.me wants to grant access to google.com", for example.