I know I'm a bit late, but I'd suggest that you put a stove, grill, and/or fireplace into the same room as the sim and have them attempt to cook. I'm not sure how fire ghosts work, but I know that the chances of a fire when sims are cooking or a fireplace is on is high. Or, I'd suggest having your sims light fireworks inside the house. That'll defintely catch it on fire. And, sadly, you'll just have to keep canceling their attempts at putting out the fire. Even when autonomy is off, sims will autonomously do things to save their lives in situations such as fire or thunderstorms. If you're still having issues, I would suggest looking up "GrayStillPlays" on Youtube and watching his video on killing off sims with fire hazards. Hope that helps (even if you're no longer doing this lol)