Modify Header Value (HTTP Headers) is an extension that can add, modify or remove an HTTP-request-header for all requests on a desired website or URL. This Addon 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 desired domains. 2. For a complete list of HTTP headers, please take a look at the list of all HTTP Header Fields on the Wikipedia page (https://en.wikipedia.org/wiki/List_of_HTTP_header_fields). 3. Clicking on the Toolbar icon opens a UI to quickly switch the Addon ON or OFF, or open the options page. 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 addon's homepage (https://mybrowseraddon.com/modify-header-value.html) and fill out the bug report form.