If postgres is too heavyweight for you but you still want client-server, I'd consider MySql. It's an old classic, pretty fast and scalable, and has much better mainstream support and a bigger ecosystem than Firebird.

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.