Has anyone gotten Auth0 to work with MERN? I couldn't find a good working tutorial for it.

stormpath.com/blog/build-a-react-app-with-user-au… That will help:)

Try PassportJs, and I think mern-cli provides templates with authentication

I am using FireBase with this.

I dont do MERN, but on my last React project I used Amazon Cognito user pool service. It has permanent free tier for 50.000 (active!) users per month and I can tell you I'm not writing any more custom code for authentication, login/register/forgot password/email confirmation, etc. Haven't used it, but you can also do federated Identites easily.