You must first research:
New Fighter Craft.
Prerequisites to even having this as an option are owning both a Alien Nav Computer and a UFO Power Source. Both can be found in alien UFOs (wether downed or landed).
Also, in order to research this you will also have to consume:
75 Weapon Fragments
Once the research is complete, your Engineering will be able to build the Firestorm. Below are the minimum requirements to build this and claim your achievement! Of course, like most things you build, the more engineers you have, the cheaper the item will be.
Research: New Fighter Craft
Alien Alloy: 60
UFO Power Source, UFO Flight Computer x2
Days to Build:14