Examine source code of FB unseen

{
  "background": {
    "scripts": [
      "js/lib/store.js",
      "background.js"
    ]
  },
  "browser_action": {
    "default_icon": "icon48.png",
    "default_popup": "/popup.html"
  },
  "content_scripts": [
    {
      "css": [
        "css/read_button.css"
      ],
      "js": [
        "js/lib/jquery-2.2.4.min.js",
        "js/lib/relaxed-json.min.js",
        "js/read_button.js"
      ],
      "matches": [
        "*://*.facebook.com/*",
        "*://*.messenger.com/*"
      ],
      "run_at": "document_end"
    }
  ],
  "description": "Blocks the ”seen” feature of the facebook messenger, and your friends will no longer be able to see if you read their messages.",
  "icons": {
    "16": "icon16.png",
    "48": "icon48.png",
    "128": "icon128.png"
  },
  "manifest_version": 2,
  "name": "Unseen",
  "permissions": [
    "webRequest",
    "webRequestBlocking",
    "*://*.facebook.com/*",
    "*://*.messenger.com/*"
  ],
  "version": "2.1resigned1",
  "web_accessible_resources": [
    "*"
  ],
  "browser_specific_settings": {
    "gecko": {
      "id": "{230ed5ec-936c-4ad1-b3d4-e2bb251bd1c3}"
    }
  }
}