>I have an SQL script that inserts a few thousand rows into my Firebird
> database.
>
> It will run when nobody else is connected to the database and when
> there are no other transactions running concurrently.
>
> Execution time does NOT matter.
>
> Is it better to COMMIT every single insert?

Commit every insert only if you have good reason to do so.
(From your short description I would say NO)

Ivan