For a first game, pretty good! It's a simple, but well-made project. I like the fact that the background changes colour every now and then and the little shake whenever you kill an enemy which makes it more enjoyable.
Although it's a bit pity the gameplay doesn't change much. The enemies get faster and appear more often – great!, but there's no new gameplay element introduced as the game progresses. Another weapon, temporary power-up, new kinds of enemies, a boss – you name it.
Anything which would make me believe game at score 2000 would play differently than a game at score 1000 or 500.
As for the details:
– I think the animated title in menu scene looks a bit wrong (jaggy movement with a sudden skip at the end, plus the outer glow is cut off at sides
– Since you have a fullscreen switch in the options, you don't really need the footer (which is cut off in half because in the Project System, you need to add 38 to the vertical resolution of your game). In Unity, you can build the game with a Minimal template to get rid of it.
– "Quit" is pointless in a WebGL game since all it does is freezing the game (it can't close your browser or anything)
– The black background (1000+ score) makes it hard to see the crosshair
But I'll repeat – for a first game, this is actually quite good. Welcome to Newgrounds!