Modify Header Value is a Firefox add-on tailored for web developers and testers, enabling them to add, modify, or remove HTTP request headers effortlessly. This subscription-based tool is designed to simplify the testing process for app developers and web designers by providing a user-friendly interface to manage header items. The add-on also includes a tutorial on customizing headers, making it accessible for those new to HTTP requests.

The functionality of Modify Header Value is extensive, featuring a comprehensive list of HTTP headers available for user selection. It allows the addition of multiple header items, which can be managed individually to test various scenarios effectively. The toolbar icon enhances usability by providing quick access to enable or disable the add-on, making it a standout tool for anyone involved in web development.