>> And where can I define this format? In the firebird.conf? where?

This are format's for sending data values to the server, ie, Pavel was
talking about the formats you can send to the server in a INSERT or UPDATE
statement.

You don't need to config nothing anywhere, just use one of this valid
formats.

What I think you are looking for is a way to format the output from the
server, and Daniel Rail has already posted the valid answer for your
question.

Artur