Examine source code of Headers

{
  "manifest_version": 2,
  "name": "Headers",
  "version": "1.3",
  "description": "Simple extension for HTTP headers manipulation",
  "homepage_url": "https://github.com/ZygmuntJakub/Headers",
  "icons": {
    "32": "icons/icon-32.png",
    "48": "icons/icon-48.png",
    "128": "icons/icon-128.png"
  },
  "permissions": [
    "webRequest",
    "storage",
    "webRequestBlocking",
    "<all_urls>"
  ],
  "browser_action": {
    "default_icon": "icons/icon-32.png",
    "default_title": "Headers",
    "default_popup": "popup/index.html"
  },
  "browser_specific_settings": {
    "gecko": {
      "id": "headers@zygmuntjakub"
    }
  },
  "background": {
    "scripts": [
      "background/index.js"
    ],
    "persistent": true
  }
}