I am certainly not starting a firebird flame all over again, but if you want interoperability with mainstream other applications plz do not choose Firebird for embedded. Use Sqlite for embedded.
Otherwise: if that's not important, firebird is an option as an embedded solution.
For larger projects I would never ever go with Firebird, because it is definitely not a mainstream solution and might get you into trouble with maintenance and interoperability with other software, like f.e. spreadsheets. These do not have a clue (ODBC if and when a driver is available for your platform, as partial solution) what the Firebird format is.... They have about mainstream solutions.