Actually, it is entirely safe, if done right. fill a tub with hot water, and lower a pot of petrol into it. The flash point of fossil fuels is quite high, it is when the vapours come into contact with fire that you have problems. However, the common recipie that involves heating creates a very unstable form of napalm-a that sweats a clear, thin liquid that is highly reactive. So don't make it if you don't plan on blowing it up quickly, it is dangerous to store.
But gasoline(petrol) isn't nearly as dangerous as people think. You can put a cigarette out in napalm-b. You can throw matches into a barrel of gasoline. Just mind the vapors.