An ability that looks for whether a creature “had to attack” checks to see if there were any requirements to attack that were applying to that particular creature at the time it was declared as an attacker. If an ability required that creature's controller to attack rather than the creature itself (such as that of Trove of Temptation), that creature dealing combat damage won't cause Firkraag's last ability to trigger, even if it was the only creature that player could have attacked with to meet the requirement.