> Is it possible to nest transactions with the same IB_Transaction
> component?

The nested transactions in Firebird are called SAVEPOINT. You can set
savepoint using "normal" query

SAVEPOINT savepoint_name;

If you need to rollback the nested transaction, you execute

ROLLBACK TO savepoint_name;

If you no longer need the savepoint (in other words you're sure that
you won't rollback to it), you can release the savepoint using

RELEASE SAVEPOINT savepoint_name;

All these queries are executed using normal query component that is
assigned to your main transaction.

Roman