Modify Header Value can add, modify or remove an HTTP-request-header for all requests on a desired website or URL. This Firefox add-on is very useful if you are an App developer, website designer, or if you want to test a particular header for a request on a website.
Key features:
1. Easily add, modify or remove a header for any request on a desired domain(s).
2. For a complete list of HTTP headers, please take a look at the list of
HTTP Header Fields in Wikipedia.
3. Clicking on the Toolbar icon allows you to disable or enable the addon.
4. Each header item has several options to adjust (see add-on's Options page for details)
5. You can add several header items and active or inactive them separately.
6. An example in the options page shows a tutorial on "How to add a custom header".
Note: to report bugs, please visit add-on's
homepage and fill the bug report form.
Review on YouTube:
https://www.youtube.com/watch?v=KM4uKx-BjIg 
