Examine source code of User Agent Switcher

{
  "description": "Change user agent and allow custom user agent based on domain",
  "manifest_version": 2,
  "name": "User Agent Switcher",
  "version": "1.0",
  "icons": {
    "48": "icon-48.png",
    "96": "icon-96.png"
  },
  "permissions": [
    "webRequest",
    "webRequestBlocking",
    "<all_urls>",
    "storage",
    "tabs"
  ],
  "background": {
    "scripts": [
      "background.js"
    ]
  },
  "browser_specific_settings": {
    "gecko": {
      "id": "non-existing-email-for-addon@thewheatfield.org"
    }
  },
  "browser_action": {
    "default_icon": {
      "19": "icon-19.png",
      "38": "icon-38.png"
    },
    "default_title": "Choose a user agent",
    "default_popup": "popup.html"
  }
}