Examine source code of GreenifyFacebook

{
  "manifest_version": 2,
  "name": "GreenifyFacebook",
  "description": "Change the blue Facebook into a green Facebook",
  "version": "1.9",
  "content_scripts": [
    {
      "matches": [
        "https://www.facebook.com/*"
      ],
      "js": [
        "content.js"
      ]
    }
  ],
  "icons": {
    "128": "icon.png"
  },
  "browser_action": {
    "default_icon": "icon.png",
    "default_popup": "popup.html"
  },
  "permissions": [
    "storage"
  ],
  "applications": {
    "gecko": {
      "id": "@GreenifyFacebook"
    }
  }
}