Hello, I just completed Firework. It was a great game with a lovely twist at the end. While playing, I noticed a few grammatical errors, and while I can't remember the exact lines I do remember the errors:
When you're about to fight the second boss he says something along the lines of "Defective ones like you really makes me furious". The "makes" should be "make"
I can't remember the line but it's while you're speaking with the boss trio after defeating all three of them. The error occurs while speaking to the girl, she says "catched up" in her line and it should be "caught up"
I'm not sure if there is an actually an error here or I'm misremembering but at the end while speaking with the final boss "strength" is misspelled as "strenght"
Aside from those I have some suggestions that might help gameplay:
When acquiring your first item from a boss, add a tutorial message showing how to swap between items. When I received the shield from the first boss, I assumed it was some sort of passive and I didn't have to equip it. Speaking of the shield, I also never figured out how to use it.
Possibly make the items passive abilities in the first place. I'm not sure if that would break the gameplay, but the only item I really used were the arrows you get from the third boss. I never used the shield, and I only properly utilized the smoke once, and it was unintentional.
When the second boss "heats up" I couldn't see his remaining health. I'm not sure if he had a second health bar like the last boss or that was a bug. I also couldn't see his health when fighting the trio.
In the penultimate level, there are blue zones that propel the player around in various directions. However, one of them simply lowers the gravity, and it looks the same as the one that shoots the player upwards. It's not a huge deal, but I would recommend distinguishing the two of them through something simple like a slight recolor.