Massive forest/brush fires, particularly in places like California where considerable property destruction and even loss of life could occur, would make plausible terrorist weapons. However, attacks against our electric and communications infrastructure might be more effective at crippling the US economy. Consider the effects of of coordinated attacks on major portions of the US electric grid or against microwave towers. Massive east coast blackouts have happened in the past caused by just a single equipment failure triggering a cascade of overloads and shutdowns.
Forest fires are an attack on our electric grid. High voltage transmission lines often run through forests.