Firebird mentioned in two articles

IT-Director.com – A phenomenon of Open Source has been that a single product normally emerges in any given category. Firefox (browser), Linux (OS), Apache (Web Server) and JBoss (middleware stack) are all examples. The only market where more than one product seems to have emerged is database—which is dominated by MySQL, but also served by FireBird, SleepyCat, Ingres and others. – Read more here.

ZDNet – MySQL is not alone in that thinking. Other established open-source databases include PostgreSQL and Firebird. – Read more here.