HTTP Header Spy is a Firefox add-on designed for web developers and IT professionals that provides a streamlined method for inspecting HTTP request and response headers directly within the browser. This tool allows users to view essential header information, including cookies and response times, without needing to navigate to the page inspector tool. Features include auto-hide information panels, server IP display, and the ability to follow full redirect paths, making it a practical choice for efficient web development tasks.

In addition to its core functionalities, HTTP Header Spy offers a range of customizable options, such as choosing between light and dark color schemes, filtering headers, and highlighting specific string patterns. Users can also utilize a micro mode to focus on selected response headers, enhancing the inspection experience. The tool's user-friendly interface allows for easy single-click header highlighting and the option to position the display in one of the four corners of the browser window.