How can you tell it works? I've been trying now 5 times to enable it, and testing if it works. If I understand correctly, if I log in to gmail.com (mail.google.com), google.com should be logged in, but google.dk and youtube.com shouldn't since First-Party Isolation should be isolating them, but no matter how hard I try, it doesn't work. If I log in to mail.google.com, I get logged into youtube.com, google.com and google.dk.

Am I misunderstanding how it is supposed to work?

I've tried completely uninstalling firefox 5 times now - including wiping the profile from my machine - but the same thing keeps happening.