Back in 2000, Borland (I think they may have been called Inprise at that stage) decided to opensource Interbase (version 6 I think) , which is where Firebird comes from. Shortly after, however, Borlard reversed their decision and took Interbase back in house again. Firebird remained active.

In a previous life I was a Delphi developer, and used Firebird almost exclusively. It was an amazing database - client server, yet fast with very small resource requirements and as good as no admin requirements. Set and forget. I really don't know why it hasn't had more widespread adoption, but some of the problem seems to be due to lack of good drivers