I had a fire start from a fireplace was when one of my played households went to another played household's house that had a fireplace going when I played them last. That was the only time in over 1800 hours of game play where I have seen a fireplace fire. However, I usually have my sims level up their practical skills (cooking/handiness/charisma) right away when I start out, that and not all my households have fireplaces, so that may be why.
I don't think any sims that lived there with the fireplace had handiness, now that some simmers have mentioned it. I didn't realize it could be related. I would try a sim with no handiness in a bad mood with objects close to the fireplace if you wanted to try to start one...but of course us simmers would never do that on purpose... >:)
On a side note, I don't think urns or tombstones can catch on fire. They're already hard enough to delete on purpose.
Let me know if you start a fireplace fire, I'd like to know the conditions that would start one.