Sorry, but if you slow the video to 0.25x and watch closely, you can see it's a firefly :P
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.