> Ok, let me know if this works for you:

Confirmed.

Seems to be Firebird's specifics - when database is created in dialect
1, it does not really matter what client dialect is passed - server
always return the column type as SQL_DOUBLE (value of
XSQLVAR.sqltype=481). When database has dialect 3, server returns the
column type as SQL_INT64 (XSQLVAR.sqltype=581).

So this is either bug or feature of the Firebird, Jaybird correctly
translates what is returned from the server.

Roman