Examine source code of confirm-mailto

{
  "manifest_version": 2,
  "name": "confirm-mailto",
  "version": "1.1resigned1",
  "description": "Confirm with users before opening any mailto link on the page through their default email client.",
  "applications": {
    "gecko": {
      "id": "jid1-SMDkP5AvcbEetw@jetpack"
    }
  },
  "content_scripts": [
    {
      "js": [
        "jquery-3.2.1.min.js",
        "confirm.js",
        "application.js"
      ],
      "matches": [
        "<all_urls>"
      ],
      "run_at": "document_end"
    }
  ]
}