Header Modifier: Edit HTTP Headers by TechyTools

Header Modifier lets you add, change or remove HTTP headers, and block or redirect URLs, in Firefox. Test APIs with an auth token, switch environments, browse as a phone, swap a production script for your local build, or fix CORS while you develop, all from one small popup.

FREE, WITH NO LIMITS
• Unlimited request headers: set, append or remove
• Turn each header on or off, or pause everything with one switch (Alt+Shift+H)
• Filters: limit headers to domains, URL patterns, regular expressions, or the page making the request
• Exclude rules, for health checks, CDNs or anything you want left alone
• Choose request types (page loads, fetch/XHR, scripts, images and more)
• Block requests by URL pattern
• Redirect URLs to a fixed address, for example a CDN script to your local build
• Presets: browse as an iPhone, Android phone or Googlebot, skip caches, send a bearer token, and more
• Value suggestions for common headers like Content-Type, Accept and Cache-Control
• One-step import of profiles exported as JSON from other header extensions, including URL redirects
• The toolbar badge shows how many headers and rules are on
• Light and dark mode

PRO (optional, coming soon)
• Switchable profiles for each environment
• Response headers: CORS, Content-Security-Policy, caching, Set-Cookie
• Rewrite URLs with regular expressions ($1 groups), and add, change or strip query parameters
• Lock a profile to a single tab
• Keep profiles running on their own domains while you switch others
• Export and back up your setup

BUILT TO BE TRUSTED
• No analytics, no telemetry, no ads, no affiliate links
• No remote code: everything runs from the package itself, unminified
• Your headers stay in your browser's local storage
• Uses Firefox's declarativeNetRequest API, so the extension never reads your traffic. Firefox applies the rules.
• Never for sale. This extension will not be sold or transferred to another owner.

SITE ACCESS
Firefox only lets an extension change requests on sites you approve. Allow all sites, or only the domains you work with. When one site calls another (an app on localhost calling api.example.com), allow both.

Guide: https://techytools.in/header-modifier/

Header Modifier is made by TechyTools, a brand of Netavi Systems Inc. (British Columbia, Canada).