Natalya Polyanskya, last update 09-May-2012
As you probably know, Firebird database can be installed and run at Mac: this is official Firebird SQL page at Apple's site.
Despite the fact that page at Apple says only version 2.1.1 is available for Mac, actually there is the most recent Firebird 2.5.0 database version available for Mac, as well as previous release Firebird 2.1 (scroll down for Mac OS X distribution of Firebird database).
Firebird is an excellent database for Mac - it's reliable, silent, full-featured database engine. It's important to say that Firebird database at Mac is fully compatible with Windows and Linux versions - there is simple 1-step migration from/to Mac to/from Windows and Linux.
There is a bit outdated, but still useful short introduction how to setup Firebird at Mac.