One other cause of coop fires that I never see mentioned is the dust. Yep, good old dust. It accumulates everywhere, especially when the coop is more closed up in the winter and the chickens are spending more time in there. That dust is insidious - it gets everywhere, including on the prongs of the electrical plugs that might not be pushed all the way in. Haven't we all been guilty of having the prongs pushed most of the way in, or having prongs that go in a little cocked to one side? Imagine dust and dander getting into that tiny gap, or filtering into the outlet itself. That dust and just a little humidity can cause arcing across. And that dust accumulates on the surface of the bulbs as well. Many of us dust things off during the summer when we spend more time out there, but how many of us want to stand in the coop dusting things off when it's already getting dark at 4:30 and it's 20 below?
I have a light in my coop that is very seldom used. It's hard wired in like the rest of the electrical system out there is. It's there for my convenience if I have to go out in the early darkness and do something out there. My husband is an electrician. He's no dummy, either - he was the electrical engineer on board both an aircraft carrier and the Battleship USS Wisconsin, and he assisted in writing the Navy's electrical tech manual for the IC/EM courses. We went out a couple of weeks ago to check on Scout, our chick who had frostbitten feet, and I flipped the light on. A few minutes later there was the smell of something burning. Ken looked up and said, "Get that light turned off ASAP!" Like a dummy I had to look first and flip second. The top of the light bulb was covered in dust, chicken dander, and probably feathers or down, and just before the light went out we could see a little, almost invisible wisp of smoke coming from the top of the bulb. He whipped out the little mag flashlight he always carries (with his ever present Leatherman as well!) out of it's holster on his belt and made sure that there was nothing really seriously wrong aside from scorching dust. Only when he was sure did we finish what we were doing and go back inside.
That could have caused a real problem if a bit of smoldering feather had drifted down into the straw and litter covered floor. A heat lamp is out there to provide heat. It won't keep the coop at the same temperature as your living room, but that bulb gets hot, hot, hot. And dusty. Yikes! Now I know that a whole lot of things have to go wrong for a fire to start from a bit of dust on a light bulb. I'm not one to spend a lot of time with couldda, wouldda, shoulda. And I don't go looking for trouble where there likely isn't any. But I'm darned if I'm going to be the tester for the theory that the dust and feathers are combustible and the bulb is a heat source.

None of us are as diligent about small housekeeping projects out there in sub-zero temps. Accidents are a series of things that have to happen during an event, and I personally believe that a heat lamp doesn't have to come tumbling down onto the floor and sit there for a time in order for there to be a bad one.
And by the way, it was JackE who gave me advice on winterizing my coop, and I argued with him vehemently about it. Um, he was right. We live in Northern Wyoming. Last year our last snow flurries of the year came on June 6th. My coop isn't insulated or heated, and my birds are just fine. Got 8 eggs out of 10 chickens today, and they are active, healthy, and heading outside for some exercise the minute the sun comes up - and on some wintery days even when the sun doesn't come out all day. Thanks for that dose of common sense, JackE! Not often I'm happy to admit publicly that I was wrong, but I was wrong!