The controller of the ability chooses one of their opponents. If this card changes controllers before the ability resolves, you still pick an opponent of the player who controlled it when it was activated.

If you activate this multiple times in a row so the stack has more than one of this ability on it, the abilities will still resolve even if Goblin Festival leaves the battlefield or leaves your control before they resolve. For example, if activate the ability twice, and after the first flip you lose control of it because you lost the toss, or it leaves the battlefield, then you still flip for the second ability.