Temporal Firestorm

The kicker ability doesn't let you pay a kicker cost more than once.

To determine a spell's total cost, start with the mana cost (or an alternative cost if another card's effect allows you to pay one instead), add any cost increases (such as kicker), then apply any cost reductions. The spell's mana value remains unchanged, no matter what the total cost to cast it was.

You may kick Temporal Firestorm only once for its {1}{W} cost and only once for its {1}{U} cost. You can kick it twice by paying {2}{W}{U}, but you can't kick it twice by paying {2}{W}{W} or {2}{U}{U}. You can't kick it more than twice. Notably, this means that X will always be either one or two if Temporal Firestorm was kicked.