Examine source code of HHSwitcher

{
  "description": "Adds browser action icon to toolbar to choose user agent string from popup menu. UA include Firefox 53(Mac), Chrome 59(win7), Opera 12, Safari 7(Mac), Edge 12(Win10), IE 11(Win8) ",
  "manifest_version": 2,
  "name": "HHSwitcher",
  "version": "1.2resigned1",
  "icons": {
    "48": "icons/agent-48.png"
  },
  "permissions": [
    "webRequest",
    "webRequestBlocking",
    "<all_urls>"
  ],
  "background": {
    "scripts": [
      "background.js"
    ]
  },
  "browser_action": {
    "default_icon": "icons/agent-32.png",
    "default_title": "Choose a user agent",
    "default_popup": "popup/choose_ua.html"
  },
  "browser_specific_settings": {
    "gecko": {
      "id": "{039fa692-64c9-4f34-99a0-a08b96187468}"
    }
  }
}