Possible, but if they can't cause a loss of life, they will lose face.
It would show that we are beating them so badly that they are only able to make demonstration gestures back at us.
Yes, they can start forest fires. Those fires are unlikely to kill many Americans, however, and they won't be as large as last year's fires simply because we've built new logging roads as firebreaks over the last 12 months.
Frankly, what we've seen them attempt so far with Ricin in the failed London attack, Sarin and Mustard in the roadside bombs in Iraq, and VX in the failed Jordanian attack, points to a trend that shouldn't be overlooked: a poison attack.