1. How do you know it's a firefly? It's just a bright spot.

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.

I'm pretty convinced it's a firefly after watching the video frame by frame. Go to the YT video, set it 1080p and pause at 36s. Then go frame by frame (use ',' and '.' to go back/forth by a single frame) and you can see it appear as a dark blob, that comes down, eventually lighting up, then contrary to your point 3 it actually goes in front of the trees.

I did go frame by frame myself earlier. The fact that it starts as a dark blob is pretty good evidence for it being a firefly, and I think I saw it earlier as part of the dramatic increase in diameter, but I can't see it that way right now.

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.

Watch it at 1080, you'll see.... Or ball lightning starts off as a black blob.

I saw it too, it starts as a dark dot at the top of the cloud.

I didn't notice it until you said it but now even at full speed it's obvious. You can see it fly in before it starts glowing.