Thanks for the review! As tested on Firefox Version 122, the add-on actually gives windows different colours based on the active tab in them. I must apologise but the add-on not working on a per-window basis might be related to the browser verison. Feel free to open an issue on GitHub page if the issue occurs on the current release version of Firefox.

For a smoother colour transition, you can refer to this thread: https://github.com/easonwong-de/Adaptive-Tab-Bar-Colour/issues/43. TL;DR: It's recommended to use a CSS theme for that purpose.