How mature is the Firebird database when compared with PostgreSQL and MySQL? I have used PostgreSQL with small projects and MySQL is running in production environment. No issues with them. I have been hearing of Firebird which is again a solid RDBMS in the open-source area.

How reliable, ANSI-SQL compliant and stable it is when compared to PostgreSQL? How
FirebirdSQL is quite mature and reliable. 

Its biggest weakness however (in my opinion) is the lack of a proper manual that covers all features of the current release. Currently if you want to know if a specific feature is available you first need to go through the Interbase manuals, than through the Firebird 1.0 release notes than the Firebird 1.5 release notes, than the Firebird 2.0 release notes and finally the Firebird 2.5 release notes (and 3.0 is more or less on the doorstep)

Btw: using a different DBMS in development than in production is usually a very bad idea. Especially if the development DB - as in your case - offers a lot more SQL features than the production DBMS.