This repository is a comprehensive collection of CSS hacks and stylesheets for customizing the appearance of Mozilla Firefox, primarily through userChrome.css and userContent.css files. It provides over 200 self-contained CSS files that modify various UI elements such as tabs, toolbars, sidebars, URL bar, context menus, and more. The repo also includes a Python script (add_style.py) for managing tags and creating new styles, and an interactive HTML/JS tag browser to explore styles by category. The collection has been actively maintained for years, with styles updated to match Firefox's evolving internal structure, and a deprecated folder for older versions. While each CSS file is tightly coupled to Firefox's internal IDs and structure, the overall organization and modular design make it easy for users to compose their desired appearance. The project is not a standalone application but a resource for Firefox theming enthusiasts.