I've been working with PostgreSQL, Firebird, Oracle, MySQL, MSSQL more than 6 years ago and my preferred are Firebird and PostgreSQL. I don't like MySQL because Commercial License is not cheap, and MySQL lacks of some functions.
I you ask me about what is my preferred one between FB and Pg.... It varies about requirements and needs. I feel that Firebird works better when speed is a must. I've been working with very large databases environments and I have not had problems. PostgreSQL is better in web sites and internet applications developed in PHP, Python and Ruby, but I think that this will not be for much time. In the last months big companies are looking to FB as a very serious DB. I recommend you FB. Try it, give it a chance, you won't be disappointed.
