Link below is the most genuine post-mortem for Persona you'll read. It's a shame Persona didn't work as it's a great concept, but was always going to be a vast challenge for Mozilla, especially in the middle of launching a competitor to Android+iOS.
tldr; we couldn't get it to work.
Persona was never close to being shippable on desktop. It's true that we spent effort trying to make Persona work for Firefox OS, and that effort did not result in a fantastic on-device experience. Sign-in to web? Yes. Sign-in to device? Not so much. Federated login is really hard, unsurprisingly, for UX reasons as much or more than raw technology reasons. This is difficult stuff, and changing user expectations about how an "account" works is very, very difficult.