I think that’s how it works. I’m not 100% sure but I remember it being explained that once a user visits a community from another instance for the first time a copy is made and only new posts start to populate the copy. Meaning the past discussion is not ported. Also, I assume the users are low because it shows people subscribed in that copy just made but not including the other instances’ subscribers.
The only thing keeping me from Firefox is native tab groups, I use that all the time.