Excellent game, super good art too. You got the level design right as well. Its only problem is that it's too short. I may be a bit biased because I love scrolling shooters. Space Invaders, Galaga, Xevious, Gradius, and the parody games Chicken Invaders, Parodius, etc. On my first run my score was 1795300. It would be very cool if you had (or have you?) inserted some secrets/easter eggs, like secret things that can blow up.
Why is firefox nightly required? Did you use a particular feature that is cutting edge or is the set of tools requiring it?