Can you start forest fires using a drone? To answer this question that you already know is ‘yes’, lets ask a couple other questions? Can drones carry equipment? There are agraculturaal drones that can spray pesticides, carry UV or other spectrum equipment to survey fields for hot spots, and most obviously, heavy camera equipment. Though, besides batteries, any liquid is most likely the heaviest cargo. Can a consumer drone carry a weapon like a handgun? Yes. It has been done and it is a extremely illegal thing to do.
But these most be expensive, specialized drones
No. A consumer drone like a DJI can carry all of these. Besides the specific camera equipment adapters, other customized adapters must be retrofitted. OK, fine. So lets see how difficult it is to make a flame throwing drone. It must be really difficult.
How do you make a flame throwing drone?
Most drones can carry a gallon of liquid for a few minutes, a half a gallon for twice the duration and it scales out linear. A basic drone can fly 20-25 minutes; maneuvering, climbing and descending. Most drones have a standard range of 1/2 to the more common 2-5 mile range now with increased flight times. Drones that use directional antennas can have a longer distance for the drone itself and camera which uses a separate signal. Most drones have lights that flash and can be turned on and off like a switch.
So what do we have for controls? Drones like to stay level (quad copters). So simply accelerating forward will not tilt it enough to dump something. But, a camera pan can and the lights are a light switch or switch. There are sensors that detect light and can activate a servo that will open or close (push a trigger, open a valve, or ignite a lighter). Or simply running the fuel through a spark will do it or a constantly light torch lighter will run for 5-10 minutes.
A consumer drone can carry a flammable liquid, dump it, and ignite it. With very little adjustments, or just with duct tape. A camera movement can simply lower a straw, it does not need to move a container.
Why would people, agents, or a state want to start fires?
Take a look. It is chaos. Billions in damages. Massive resources in people and equipment deployment. And, removing that equipment from the regular use to respond to standard emergencies.
This could be done easily on a massive scale, for a mere tens of thousands for many drones. Doing this in concert, and specific times with these same resources are needed elsewhere simply overextends capabilities which creates massive delays and decreases performance. Resource exhaustion is a fatal situation.