To build it from scratch yes, it's possible to build a primitive bomb from stolen refined plutonium. After the collapse of the soviet union, a lot of it went missing and there a dozen cases of people getting caught trying to sell it on the black market (and those are just the people that've been caught.)

Mass arson is possible, though not on the scale of firebombing. The US in WWII developed the idea of strapping incendiary bombs on bats and releasing them in a city, where they would roost in random buildings and start hundreds of small fires.