This repository is a curated collection of modular CSS hacks for customizing the Firefox browser UI. It is not a standalone application but a library of CSS modules designed to be imported into Firefox's userChrome.css or userContent.css. The architecture is highly modular, with each CSS file representing a self-contained hack that can be mixed and matched via @import statements. A sophisticated system of CSS custom properties (--uc-*) allows users to configure behavior without editing the core files. The repository includes well-crafted supporting tools: a Python script (add_style.py) for managing the collection's metadata, a tag-based web browser UI (index.html, selector.js) for discovering hacks, and a reusable syntax-highlighting code block component. The codebase is actively maintained, with a clear deprecation strategy for outdated hacks. While the individual CSS hacks are tightly coupled to Firefox's internal DOM structure and are inherently fragile, the architectural patterns and supporting tools demonstrate high craftsmanship and are genuinely reusable across projects.