Hmm. The question is where is the balance for that threshold? Seems like you would quickly get no fires again once you got fire fighting buildings in place. The only reason the fires can be frequent is there is nothing to counter it until the Town Well. I know there are forges and factories that increase it later but most can be countered by the other anti-fire buildings.
I have been thinking that perhaps an auto-build building could be made if you had access to the river or ocean that gave a small anti-flammability. That would at least reduce some prehistoric era problems. But not help people who did not settle near a body of water.
I think the issue is that there are so few things that CAN result from flammability early on (basically the one burn event and not much else), so it cannot possibly be progressive. That's why I'm suggesting splitting the event into multiple events that all look the same to the user but have increasing probabilities of occurring (makes higher flammability progressively more likely to trigger a burn down, rather than it being basically an on/off thing as it is now). This approach has the benefit of not requiring any new code, or buildings.
However, I also like your idea of an auto-build small anti-flammability building that comes with a water source. Obviously they are not mutually exclusive.