When turned on, burns every unit that touches it
Once a Firestorm Generator was built these wall sections had to be constructed around the area that it was intended to defend, usually the GDI base the generator was built in. The more wall sections that a base had, the more power it costs to maintain them. The wall sections were the best base defense during TW2 and the Firestorm Crisis due to it could block enemy attack while non-blocking allied fire, could destroy any ground or air unit and, sometimes, stop incoming missiles; but it maintaining cost was expensive and needed to take a time to recharge after being used for a while.
Firestorm walls were not seen in TWIII, although new support powers that protected buildings from explosives or capture - Sonic Repulsion Field for GDI and Laser Fencing for Nod - were introduced in Kane's Wrath.