Examine source code of Get Username

{
  "manifest_version": 2,
  "name": "Get Username",
  "version": "4.0",
  "description": "Adds a button to all webpages matching instagram.com/{username}/*",
  "icons": {
    "48": "icons/logo-48.png"
  },
  "content_scripts": [
    {
      "matches": [
        "*://*.instagram.com/*/*",
        "*://*.instagram.com/*?*",
        "*://*.instagram.com/*"
      ],
      "js": [
        "getUsername.js"
      ]
    }
  ],
  "browser_specific_settings": {
    "gecko": {
      "id": "{76a3f3b9-1dc3-4084-87cb-fff751a46618}"
    }
  }
}