Interesting. I got a new MBP and moved my Firefox profiles folder across, and wow, I log into Gmail and youtube without the thing triggering the whole 2FA prompt which it should for new devices. On Safari when I did the same without moving bookmarks, no banana, had to authenticate on my Samsung device. Would have migrated all my stuff except I have 1.3TB of stuff here and that won't fit on Apple's 1TB SSDs.