I have to disagree with your observation.  It is clearly stated in the Firebird SQL Reference Manual that the development of Firebird-SQL is with a strict implementation of the SQL-92 Standard.

This is not to say that Firebird-SQL does not go beyond the SQL-92 Standard but it is somewhat limited in this respect when compared to the other major SQL implementation for which I have used them all with the exception of of IBM's DB2...