Then, I did not hear of it again. Now, this. I have a couple of observations.
Branding this as "Mozilla Persona" makes it look like all the other corporate "one-time" login things out there. Apple ID, Microsoft Live, what have you. I don't need one more account controlled by a corporation. Mozilla looks like a corporation to me (maybe it is not, but it looks like one). I do not automatically trust everything with the Mozilla brand on it.
Adding Persona to a website is not that easy, from the developers' standpoint. I looked at it for a while, and couldn't be bothered. From the server side, it's just so simple to authenticate users with passwords (or access tokens derived thereof). It would be wonderful if the browser could just take care of everything, if all users really need is a way to securely authenticate. Currently, it seems to me like password managers are winning this game.
I'm hoping to see something where password managers are extended with protocols to allow more data and access to be shared to services I really use.