WebSniffer - WebSniffer is an online tool to outlook HTTP request and response headers.
ViewHTTPHeaders.com - View HTTP header response for any web page.
Modify Header Value (HTTP Headers) - Modify Header Value (HTTP Headers) is a browser extension that allows you to add, modify or remove a header for any request to your preferred domains.
HTTP Header Live - Displays the HTTP header. Edit it and send it.
HTTP-Repeater (HTTP/s Live Headers) (Beta) - HTTP-Repeater (HTTP/s Live Headers) (Beta) is a browser extension that lets you capture browser requests and repeat the process in a loop automatically.
Firefox Developer Tools - Examine, edit, and debug HTML, CSS, and JavaScript on the desktop and on mobile.