I was in the hallway when it happened, and immediately tripped the power out. Though directly after the initial flash that illuminated the whole house, there was this weird echoey persistence of light and sound, and both myself and my Dad could see this ball of light jumping between the light sockets along the roof for several seconds. Sort of the same effect you see when you catch a glimpse of the sun, then blink after and see the persistence of the suns shape in your field of vision. About half a second after each 'jump' it made a really loud crackling sound, and each globe was toast afterwards.
It was really strange, and Dad put it down to ball lightning. Though it's not something we often discuss because of the uncertainty around the topic in general. I often dismiss it as one of those 'maybe I remember it wrong because I was so young'.
2. The diameter of the ball increases dramatically, as if it originates from the clouds.
3. It seems to disappear behind the trees, suggesting it is far away.
This isn't to say that it isn't a firefly; I just have no idea how you can be so sure.
For me it's hard to tell whether it "actually" goes in front of or behind the trees. I was thinking if it were bright enough it could fade as it descends behind the treeline. It would be a rather large firefly if it went behind the trees, though.