How mature is the Firebird database when compared with PostgreSQL and MySQL?
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.