As ADrawingMan says, the diagonal lines make the jar look flat.. BUT I would definitely accept that as a stylistic choice. It doesn't look bad, it just stands out. It's like its more of a symbol of a jar than an actual physical jar.
What I don't quite like though is, the fireflies are all very evenly spaced from each other. It makes it look kind of artificial, like a chain of lights. I would recommand experimenting with that, have some be closer together, making small clusters, others further away. You obviously worked against an artificial look, having them different "sizes" but I feel like it's not quite enough.
I find that the jar looks a little too flat but I really like the concept.
Something about the palette and colour usage makes me imagine this to be part of a cute intro sequence to some quaint NES game. I like it.
I especially like the flat look of the jar, otherwise you wouldn't be looking at this peice for such a long thime, but the jar attracted me