Custom UserAgent String is a Firefox add-on designed for users who want to personalize their browsing experience by modifying their browser's user-agent. This tool allows users to add site-specific user-agent strings, providing flexibility for different URLs or domains. With a user-friendly options page, users can easily manage their custom user-agent strings and utilize a library of over 100 sample user-agent strings that cover various browsers and devices.

The add-on features a toolbar popup with a simple ON/OFF switch for quick activation. Users can add, delete, or disable user-agent strings as needed, making it a versatile solution for web developers and privacy-conscious individuals. Additionally, users can find user-agent strings through online resources or the provided sample list, ensuring they have access to the latest options for both desktop and mobile environments.