The question is and remains why there are sqlite3 errors when accessing a firebird FDB database.
In mORMot ORM part is based by using embedded sqlite. When you use external database like Oracle, MSSql, Firebird, etc., external tables are registered as VirtualTables in sqlite. This is in memory sqlite3 database. In this database you can mix real tables in sqlite and virtual tables in one or many external databases. You can even query between this tables in sqlite.
Error code is raised in method of registering this virtual tables in sqlite3, because ZeosLib can't connect to external Firebird database, missing fbclient or bad connection string.
@jeffp
SynDBZeos connector try to create firebird database only if you use Firebird embedded version(client), based of connection string (if you use only path to database without server/port for TCP connection). For Fb 2.5 you must use Embedded version (separate download).
If you use Server then firebird database is not created, this is by design for security reason.