How to do INSERT OR UPDATE depending of existence of primary key value?

You can do it in a stored procedure or trigger with Firebird 1.x, and anywhere in Firebird 2.x (using EXECUTE BLOCK). The query looks like this:

BEGIN
insert into t1 (id, c1, c2) values (:id, :c1, :c2)
WHEN SQLCODE -803 DO
update t1 set c1 = :c1, c2 = :c2 WHERE id = :id;
END

Field ID is the primary key in above example. It's quite simple: it tries to insert and if PK violation happes, it updates the record.

Since Firebird 2.1, you can also use UPDATE OR INSERT statement.

UPDATE OR INSERT INTO t1 (id, c1, c2)
VALUES (:id, :c1, :c2);

It automatically uses primary key columns when available. If you don't have a primary key, or want to use some other columns to match the records, you can use the MATCHING clause:

UPDATE OR INSERT INTO t1 (id, c1, c2)
VALUES (:id, :c1, :c2)
MATCHING (id);

Furthermore, if you need some more complex logic, you can also use the
MERGE statement. For detailed usage description, take a look at Firebird release notes or README.merge.txt file in docs/sql.extensions directory of your Firebird installation.