> Is it possible to have a trigger fire for an insert/update/delete only
> after all changes to the table have been committed?

No

> I'm not up-to-date on 2.0 - 2.5 so I may have missed it

You could use ON TRANSACTION COMMIT trigger with, probably,
help of RDB$SET_CONTEXT\RDB$GET_CONTEXT functions. But it
fire at transaction commit time, not after. If you need after commit -
use events.

Regards,
Vlad