Thank you for playing! I'm really glad you didn't have any mouse issues, I think it's only an issue in Firefox so if you used a different browser that might be why there were no troubles.
I agree with you about the transitions and thank you for the input! :) The screen wipe at the start of the level doesn't quite behave the way I wanted it to and I didn't have time to fix it up. The death screen is definitely too slow, it's actually possible to end it early by opening the menu with tab (or Start on a gamepad) but there's no way for a player to know that, which is a failing on my part.