Custom Style Script is a Firefox add-on designed for web development, enabling users to tailor the appearance and functionality of webpages through custom JavaScript and CSS code. This subscription-based tool allows for the addition of code snippets to specific pages or across all pages, making it a versatile choice for developers looking to customize their browsing experience. Users can easily enable or disable the add-on via a toolbar icon, providing quick access to their customizations.

The add-on supports multiple code items, each with adjustable options, allowing for detailed customization. Users can also incorporate online JavaScript or CSS libraries, such as jQuery or Bootstrap, enhancing the functionality of their custom scripts. The options page includes helpful examples and validation checks to ensure code quality before implementation.