HeaderMod is a lightweight and open-source browser extension designed for Firefox users, focusing on HTTP header manipulation. It allows users to intercept outgoing HTTP requests and customize headers on a per-URL basis, offering features like wildcard-based URL matching, and the ability to add or replace request headers. Users can define multiple header-modification rules and manage them easily through a straightforward interface. The dynamic toolbar icon provides a visual indication of active rules, enhancing user experience and interaction.

With HeaderMod, users can create, modify, or delete header rules effortlessly. The extension ensures privacy by storing all settings locally without transmitting personal data. Its functionality is ideal for developers needing to test or inject custom metadata into requests, making it a valuable tool for web development and IT professionals.