When you interact with web sites, information is sent back and forth to and from your computer. This can include details about the software you are using, the browser you have installed, the web sites you visit and much more. As this all happens in the background, you are usually unaware of just what information is being shared, but Modify Headers is a free addon for Firefox that you can use to take back control.
The plugin can be easily enabled and disabled from the status bar of Firefox so you can leave it disabled most of the time and activate it whenever you need it. Alternatively, if you are involved in web development, you may find that it makes more sense to leave Modify Headers enabled most of the time and just disable it when you want to browse the web normally.
This is certainly not a tool that everyone will want to be using on a daily basis, but it does still have its place. For site debugging, particularly mobile development of site, it is incredibly useful and it is an unobtrusive utility that can be called into action whenever it is needed.