There is really nothing fundamentally wrong with building your application
the way you have but given your design you *do* severely limit your ability
to change databases easily. If current Firebird does not fit your
application structure, then Firebird may not be a candidate for your use.
Unfortunate for both you and the Firebird team, but nothing to lose sleep over.

I don't think the Firebird team should worry about enhancing its database
to handle your design needs. Firebird cannot be SQL Server and should not try.