Howdy! I'm running this on macOS Catalina (version 10.15.5) and it says the app needs to be updated by the developer to work with my version.
Hi kittybee, there's another version in the Downloads called 'Pixel_Fireplace_64bitBeta_v0_9_1.zip' that will work on macOS Catalina. It's a Beta version but it has the essential features and commands. You can find some more info on that version here: https://hammertail.itch.io/pixel-fireplace/devlog/116290/macos-64-bit-beta-v091