Fun, but broken
This is a fun game, but if you figure it out it becomes far too easy. If people figure out to avoid the Firepower upgrade after what is visually the highest level they will keep their quick fire rate, if one collects all the shields that player is essentially invulnerable for the entire level, and every 10,000 pixels (EVERY 10,000 pixels) nets them an extra life... considering there are roughly 80-90k pixels to shoot in the levels past 10, that's a lot of extra tries. After a while one doesn't need to try - just collect shields, avoid FP, and kill oneself if they get too many FP upgrades.
Any one of these very simple changes could make the game viably hard in the later stages. Remove free ships after... say, 100k points (or make it something very difficult to achieve, like every 100k points). Keep one's FP upgrades if one dies (that way for every upgrade they accidentally collect they are cumulatively. punished for). Remove shields after a certain point in the game. Any one of these changes (or any combination, for that matter) will make the game into something that will eventually end.
Otherwise, it was pretty good.