I'm currently working on retro 3D platform shooter using Clickteam Firefly and I'm excited to share with You my tool for designing levels. It's called LVL Edit. Here's some of it's features:
I know that Firefly has it's own World Editor - but I just looove making apps with Clickteam tools.
Here's some simple FPS level done with it - just to show how to integrate "copy paste LVL Edit loading level" events.
LVL Edit is still growing. You can go here to see it's updates and what's comming in next (0.4.2) update:
If You're interested visit project's webside here:
LVL Edit Official Webside
LVL Edit v. 0.4.1