I had looked at Auth0 a while ago for a different project. They seem to be very pricey after a certain number of users. They do have a lot more out-of-the-box functionality though, and if you need all their features, it might be worth using them. They’ve also open sourced a lot of their stuff and I’ve used the Angular 2 JWT authentication library with an earlier project.
Firebase on the other hand makes it easier for us to integrate their auth with the db and all the other services they provide. It also appears firebase is much more affordable.