The following applies only for non-emergency situations:

I'd use the most difficult-to-use and/or non-consumable items first. In the case of water, that means boiling it. You haven't actually used up anything from your kit, so you're still fully equipped. If you started out with your tablets, one of your treatment options would soon be gone forever.

For fire, use the slower/more-difficult Sparklite with natural tinder now, save the lighter/cotton balls/matches for when you need a fire in a hurry.

Assume things will get worse, and save the best items for when you'll need them most.