Examine source code of Show External IP

{
  "manifest_version": 2,
  "name": "Show External IP",
  "description": "Detects and shows your current external IP address (as seen by https://showextip.azurewebsites.net/) in a convenient popup.",
  "version": "1.0.7resigned1",
  "icons": {
    "16": "icon-dark.svg",
    "48": "icon-dark.svg",
    "128": "icon-dark.svg"
  },
  "browser_action": {
    "default_icon": "icon-dark.svg",
    "default_popup": "showextip.html",
    "default_title": "Show External IP",
    "theme_icons": [
      {
        "light": "icon-light.svg",
        "dark": "icon-dark.svg",
        "size": 32
      }
    ]
  },
  "permissions": [
    "https://showextip.azurewebsites.net/"
  ],
  "browser_specific_settings": {
    "gecko": {
      "id": "@showextip"
    }
  }
}