What about few slogans that would change in time:

1. Firebird SQL server: Embedded. Workgroup. Enterprise.
2. Firebird SQL server: Everything you need and free for everyone.
3. Firebird SQL server: Stability proved by time.

Or maybe a joke?

If airplanes were built the way Firebird is, your own plane would fit
the pocket and could transport your office building to anywhere you
like. And for free.