https://firebirdsql.org has been flying under the radar in-between SQLite and full-blown PostgreSQL for decades, but if you're asking which client-server database to use PostgreSQL is the default recommendation.
I'm not really sure what Firebird is for at this point in life really. It was pretty exciting when it was open sourced in the early 2000s, before postgres became the mature beast it is, before mysql acquired something as basic as transactions, and before sqlite became the default embedded db. But then it never really went anywhere.