Migrating Firebase users to AWS Cognito

There are multiple ways to migrate users to AWS Cognito. A couple approaches are:

• Import users as batches. This will require you to take users in to the forget password flow.
• Onboard users to Cognito UserPools one at a time. This approach requires you to write some code to check against Cognito if the user exists and if not to authenticate users from the Firebase endpoint and if authenticated correctly save the credentials in Cognito UserPools using Cognito AdminAuth API.

For more details, refer the article from AWS, Migrating Users to Amazon Cognito User Pools.

You can use the User Migration Lambda Trigger.

https://docs.aws.amazon.com/cognito/latest/developerguide/user-pool-lambda-migrate-user.html