The following only works with the default user ('postgres'): gcloud sql connect --user If you try to connect using this command with any other user, the error message says FATAL: database 'user' does not exist. The workaround is to connect using the default user ('postgres'), then use the "\c" psql command to reconnect as the different user.
https://cloud.google.com/sql/docs/postgres/known-issues
Per :bobm:
cloud_sql_proxy -instances moz-fx-whatever-db-instance-name=tcp:PORT
psql postgres://USER@INTERNALIP:PORT/DBNAME