Examine source code of ipaddr_button

{
  "manifest_version": 2,
  "name": "ipaddr_button",
  "version": "1.3",
  "description": "Shows your external IP address.",
  "applications": {
    "gecko": {
      "id": "firefox_button@ipaddr.pl"
    }
  },
  "icons": {
    "32": "icon/ip_32.png",
    "48": "icon/ip_48.png",
    "64": "icon/ip_64.png",
    "96": "icon/ip_96.png"
  },
  "permissions": [
    "*://ipaddr.pl/*",
    "clipboardWrite",
    "notifications"
  ],
  "browser_action": {
    "default_icon": "icon/ip_48.png",
    "default_title": "ipaddr.pl Button",
    "default_popup": "views/popup.html"
  },
  "background": {
    "scripts": [
      "button_extension_bg.js"
    ]
  }
}