The game is really fun and the transition too the game from the intro is just awesome, how the music chances and the water starts coming down is very epic, and the way how you get one firefly at the start, and the second gives you the second one to instantly get an upgrade, to teach that mechanic is pretty awesome.
The game does get pretty repetitive after a while, so maybe some alternate obstacle thingys, like bombs that explode after a while in a 3x3 area and stuff like that.
Them the harsh black outlines on the spikes look pretty bad, maybe a lighter gray would look better, because they look kinda weird on the light ground.
Then the firefly amount for the upgrades scales way too quickly, you should make it scale faster in the beginning and slower in the end, so max you'l ever need for a upgrade would be about 12 or something. But keep the 2 fireflies for first though, it teaches the mechanic really well.