Actually, a supernova would look like an impossibly tiny point source, much smaller than even your average laser pointer. I'm pretty sure the consequences of this haven't been thoroughly explored yet.

I'm not sure the Betelgeuse supernova, at a brightness around that of the full moon, would ignite fires, but I certainly wouldn't want to gaze at it with naked eyes, even at that brightness. IMHO such a light, given sufficient strength, could permanently damage the retina in your eyes.