From Command and Conquer Wiki

The firestorm generator was a central componet to the firestorm barrier developed and employed by GDI during the TWII. The large building produced massive amounts energy that were then channeled into wall sections placed around GDI bases to protect critical infrastructures. The building itself was well armored as it would be a main target of Nod attacks, furthermore GDI would almost always place the generator within the wall sections themselves allowing the generator to be protected by it's own field.

The main draw back was that the generator consumed large amounts of a bases power to keep ready, and that most of the generators had to cool down after being on for prolonged periods of time to prevent burning itself out.