How do I share Firebase Simple Login sessions across domains?

Question

I have example.com and app.example.com. How can I log someone in from one domain and share that session with the other?

Answer 1

Firebase Simple Login sessions are persisted using LocalStorage or SessionStorage which are scoped to HTML5 origin, which is a tuple of scheme, host, and port, which is why it is not shared across subdomains by default.

That said, you can manually enable this in your application by creating a shared LocalStorage system, using methods like described here at Can HTML5 databases and localStorage be shared across subdomains?.