Really strong bones here, a few ideas:
1. It would be nice if the game assumed left-to-right loading from your roster so you don't have to go up and click a new ball each time. This is especially the case since a lot of browsers put their "exit fullscreen" button right in the middle of that area.
2. The player should just start with 3 default balls (even if you keep the loadout locked until the upgrades are purchased) so that they don't accidentally launch with a single ball after playing. Perhaps better, maybe just make "empty" slots use the default ball instead (unless you want to have them do something else on purpose).
3. The upgrades screen could use a little more obvious UI hinting. It wasn't immediately intuitive to me where I was supposed to click for the first upgrade, even with the text.
Last but not least, a settings menu for audio/music toggles would be good.