First and not least, check out Celeste's themes for the official Catppuccin ports. Your work is much appreciated, Celeste!
This one is not as capable or customizable as dynamic theme extensions that use the WebExtension APIs, but it's easily adjustable without code using Firefox Color.
Works great if you want a version with a solid active tab background rather than an outlined one, although the same can be achieved using userChrome.css and one of the official themes if you prefer that, of course.
