What's the difference between NUMERIC and DECIMAL

In Firebird, there is no difference.

In SQL Standard, NUMERIC is a more strict datatype which should enforce the declared precision, while DECIMAL can accept more numbers than declared. However, in Firebird, both types accept more numbers (i.e. they behave like DECIMAL).

There is misinformation in other online Firebird related documents regarding this, but it is rather easy to test:

create table testnum (
n1 numeric(5,2),
d1 decimal(5,2)
);

INSERT INTO TESTNUM (N1, D1)
VALUES ( 123456.12, 123456.12 );

As you can see, the INSERT statement is executed without errors and when you select you would get both values back as well.