Maurizio,

In case it wasn't absolutely clear, you can't change a value
in an after insert trigger by straight forwardly updating
new.<fieldname>. After action triggers happen -- oddly enough --
after the action. Earlier versions of Firebird accepted those
assignments and quietly dropped them on the floor. More modern
versions will produce an error if you attempt to assign a value
to the target row in an after action trigger.

If the change must actually be done after the insert occurs,
you could try something like this:

update test_table s
set s.kyriga = 'test'
where s.<primary key> = new.<primary key>

replacing <primary key> with the name of the primary key field.

Good luck,

Ann