Examine source code of HTTP Header Mangler

{
  "manifest_version": 2,
  "name": "HTTP Header Mangler",
  "version": "1.1.4resigned1",
  "author": "Peter Nilsson",
  "icons": {
    "96": "icon.png"
  },
  "description": "HTTP Header Mangler inserts HTTP headers on a per host basis.",
  "homepage_url": "https://github.com/disptr/httpheadermangler",
  "background": {
    "scripts": [
      "httpheadermangler.js"
    ]
  },
  "options_ui": {
    "page": "options.html",
    "browser_style": true
  },
  "permissions": [
    "storage",
    "webRequest",
    "webRequestBlocking",
    "<all_urls>"
  ],
  "browser_specific_settings": {
    "gecko": {
      "id": "se.patriarkatet.firefox.extensions.httpheadermangler@jetpack"
    }
  }
}