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.