Can I use the Firestorm effect at the end of the step 1 of the Resolve Global Defense (so after I've rolled the dice), but before the start of the step 2 (so before I apply the eventual effect of a Loss)?

Can I use the Firestorm effect at the end of the step 1 of the Resolve Global Defense (so after I've rolled the dice), but before the start of the step 2 (so before I apply the eventual effect of a Loss)?

Yes.

There's a timing window between rolling the dice and applying the effects (among other things, to allow Satellite Uplink to be used) and you can use any relevant tech or assets then - the only thing you can't do between rolling the dice and applying their effect is roll additional dice - you can reroll the alien die, or you can add dice to the pool to be rolled when you push your luck, but you can't add additional dice to the roll you've already made.

So, yeah, in particular, you can use a Firestorm (or any other success-generating Tech) between rolling a loss and applying it so that you still have a unit assigned to the task.

With Alien Construction, because the Scientists are exhausted but remain assigned, you can wait until you've rolled for all three Techs before deciding where to spend your free success. Technically, the same applies to Stealth Satellite, but there's no reason not to use that immediately...