Obviously there are the climate change denialists who think “global elites” have paid actors setting forest fires around the globe, but there are also reputable people saying this. When I hear arson, I hear a person starting a fire with malicious intent. Are there really massive numbers of malicious firestarters around the globe, or does arson also cover things like recklessness/carelessness, like if I burn trash in my backyard or start a campfire on the woods and it gets out of control?
Depends on who is saying it. I've absolutely seen the take that increased fires are deliberately started by them in order to prove global warming is real.
I have also seen careless cigarette tossers charged with arson (though I don't remember how successfully), which would imply firestarting from carelessness could be considered arson by some.
People who just want to start a big fire (and act on it) are pretty rare. We had one in our city last year who tried twice to set our main national park on fire before he got arrested (the pattern was very deliberate, starting fires at the bottom of hills with strong tail winds).