This is excellent, very slick. I have detected a bug however. On the level with the 8 squares each with a single firefly and a single diamond. After completing the level and entering the exit, the Fireflies can still kill you while waiting for the bonus to countdown. If this happens you lose a life and have to redo the level. Sprite collision should be turned off once the gate is entered!
Hmm, that is interesting. It's been a while since I looked at the code for this but that part of the game was written very early on so I missed that as I was getting pretty good at the game once I finished writing it. I've been meaning to update the project anyway so this will give me a good excuse. Keep an eye open for an update on the page in a week or so.