Examine source code of HeaderMod

{
  "manifest_version": 2,
  "name": "HeaderMod",
  "version": "25.4",
  "description": "Inject, update, edit, modify, override HTTP headers",
  "permissions": [
    "webRequest",
    "webRequestBlocking",
    "<all_urls>",
    "storage"
  ],
  "icons": {
    "16": "icons/icon16.png",
    "48": "icons/icon48.png",
    "96": "icons/icon96.png",
    "128": "icons/icon128.png"
  },
  "background": {
    "scripts": [
      "background.js"
    ]
  },
  "browser_action": {
    "default_icon": {
      "19": "icons/icon19.png",
      "38": "icons/icon38.png"
    },
    "default_title": "HeaderMod",
    "default_popup": "popup.html"
  },
  "browser_specific_settings": {
    "gecko": {
      "id": "{43cbaf25-2fd8-4a5b-be4d-afef18c7354a}"
    }
  }
}