To
sync Firebase Cloud buckets
to Amazon S3, you need to provide authentication credentials to Cloud SDK.
Thankfully it's easy to do thanks to the boto config file
~/.boto:
[Credentials]
aws_access_key_id = XXX
aws_secret_access_key = XXX
Given that you already installed and initialized Cloud SDK, now you can mirror Firebase Storage buckets to Amazon S3:
gsutil -m rsync -d -r gs://bucket-source s3://bucket-mirror-at-s3