My experience with fire in adventure mode has only ever been with a number of weedy ground fires that never seemed to do anything.
I only ever seem to start these by accident, by apparently picking berries or something equally random. It's some option on the local flora anyway. Is there any other way of starting a longer lived blaze than this?
Also, how on earth do I set fire to objects too? I've seen lots of stories of people throwing burning heads and spears etc at people, and catching themselves on fire in the process.
I've even read about a story where someone was weilding a burn damage sword and setting people alight with that.
setting objects ablaze is done by dropping them on the burning strawberries or whatever, also lighting the ground under a guard wil set him on fire... but he wont get pissed off at you so its a safe way to kill.