I appreciate the fact that, once the database is embedded, it doesn't need to be administered, and removes the requirement for a specific server.

Firebird SQL is an open-source solution and can run on most operating systems, such as Linux, Unix, and Windows.

I like Firebird because it's very lightweight. It's very light and powerful. I can do everything that I can with MS SQL Server, and Oracle — I can do the same things with Firebird. It has some very interesting features. For example, I can update and insert with one statement. The deployment is very easy. I can deploy it on Linux, Mac OS, and Windows — it's very easy. 

Also, the integration capabilities are great.