Firestorm is the official expansion pack for Command & Conquer: Tiberian Sun, set shortly after the Second Tiberium War featured in the base game.
Tratos mysteriously hands in his research on the artifact known as the Tacitus to the GDI Daedalus team, and while the artifact's extraction was underway, the Kodiak, the airship that was carrying it at the time, is shot down by an intense ion storm which also cuts off GDI's link to the Philadelphia space station. As the Southern Cross base assumes command and attempts to retrieve the Tacitus, Nod is found licking its wounds from the previous conflict and trying to repair and restore their main AI, CABAL. However, once that is done, it turns on its masters and tries to claim the Tacitus for its own needs.
Firestorm adds two new campaigns (one for each side) with 9 missions each. Both campaigns are entirely canonical and occur simultaneously, which is a first for the Command & Conquer series, but this time they are entirely linear and there are no side-missions like in Tiberian Sun. Both sides get new units and technologies, and some balance and gameplay changes are introduced. A new set of skirmish/multiplayer maps is introduced, as well as new Tiberium life forms known as Tiberian floaters, and new music. The multiplayer aspect of the game is expanded with the World Domination Tour (available only on the official Westwood Online server), which had players divided into GDI and Nod forces fighting for control over territories around the world, which would be periodically reset.
The main game's executable is altered in such a way that it lets the players choose whether they want to play the base game or the expansion right at the start, with the option to switch back and forth in the main menu.
