N18 How does Firestorm work ?

[TurdBuster5000]

Hey, I was wondering about the ruling of Firestorm and how to choose the target.
For example the closest target it picked in the firestorm, but if while resolving damage the first closest target dies due to said damage is another target (the next closest) then picked as the new firestorm damage target?
OR
Is it a case of all the damage is resolved at the same time and the first closest target may just end up with overkill?

 

[Stompzilla]

The latter.