> Ok, I'm doing a
> SET TRANSACTION WAIT ISOLATION LEVEL READ COMMITTED;
> in both sessions.
>
> But now when I'm doing:
> Session 1:
> SET TRANSACTION WAIT ISOLATION LEVEL READ COMMITTED;
> insert into ....
> -- sleep here, goto Session 2
> commit;
>
> Session 2:
> select count(*) from ...
> -- it hangs here until the commit

Use
SET TRANSACTION WAIT ISOLATION LEVEL READ COMMITTED RECORD_VERSION;

> Firebird should handle this also very well because it has MVCC, right?

Yes, unfortunately default for READ COMMITTED is NO RECORD_VERSION,
which basically means disabling MVCC.

Ivan
http://www.volny.cz/iprenosil/interbase/