That's shaping up as a nice little game. I like the ambition of it: boss fights, weapon upgrade shop, etc.
Thanks

- The player's bullets move annoyingly slowly. I don't know whether there's something in the upgrade shop that speeds them up. (It wasn't obvious to me what some of the upgrades actually did. If I hadn't read about the "pulse generator" above I wouldn't have guessed its meaning.)
Good point.
The "pulse generator" increases the frequency of bullets.
I think I'll add the possibility to upgrade the lasers (more speed, more damage)
- The enemy bullets are quite hard to see. How about making them bigger or brighter (or darken the background, as Egon suggested)?
Hm..I dont know what to think about darkening the background, its so nice...

I'll try to give the bullets more contrast.
If that wont work, I'll set the background darker...
Simon, in the upgrade shop you can rotate the guns.
I noticed that option, but it didn't seem to do anything. Game instructions need to be very clear to be understood by someone as stupid as me.

Good point too... a help-text in the upgrade-shop should fix that

- Collision detection could be a little more precise. It felt like the collision boxes were a bit bigger than they needed to be (although it probably doesn't matter much if the game is frantic enough).
Currently it checks the dimensions of the images (rectangle)
Technically, this is not a sroller, because there's no actual scrolling. Yes, the background slides past, but that's just visual. There's nothing in the gameplay that interacts with anything scrolling. So, it's a static shoot 'em up (like Space Invaders, except that the ship has two degrees of freedom). Just a pedantic clarification.
Would this be a good addition to gameplay? Undestroyable objects, like asteroids or space-trash, which the player has to avoid? I'll think about it. Should be fairly easy to implement...