Hi Jenny,
I believe that similar functionality to case will be available in
Firebird 1.5, however, in the mean time you can use a union:

SELECT Code FROM Supro, Person
WHERE
person.Postal1 IS NULL
AND Person.postal2
BETWEEN Supro.P1 AND Supro.P2
AND person.personid = 6789
union
SELECT Code FROM Supro, Person
WHERE
person.Postal1 IS NOT NULL
AND Person.postal1
BETWEEN Supro.P1 AND Supro.P2
AND person.personid = 6789

Regards
Ian Newby