> Looking through different Firebird related documentations (Helen's book,
> IBP pdf set, ...), it seems that Firebird doesn't support the
> SQL-compliant transaction isolation level "REPEATABLE READ". Is this
> correct?

Yes. SQL standard tries to avoid specifying any implementation
details (like indexes that influence performance but not the result),
but it fails in case of transaction isolation levels, because it describes
lock-based databases and completely "forgets" multi-version databases.
The isolation levels are somewhat similar, but not equivalent.

Ivan