I'm trying to get the list of users that created an account a week ago to send them a call to action email. Is that possible with firebase auth with the javascript library version 8?
There are no APIs provided by Firebase Authentication for this. Firebase Auth is not really meant to work like a database that can be queried.
What you should do instead is use a database to store each new user, and attach a date to that user in the record you create for them when they first sign up. You can use that date to query the database for the newest accounts.