Examine source code of Limit Tabs Four

{
  "applications": {
    "gecko": {
      "id": "dvm.quizmaster@gmail.com",
      "strict_min_version": "79.0a1"
    }
  },
  "description": "For my autistic son. Limits tabs to Four. Period",
  "manifest_version": 2,
  "name": "Limit Tabs Four",
  "version": "1.0.0",
  "background": {
    "scripts": [
      "limittabsto4.js"
    ]
  },
  "browser_action": {
    "default_icon": {
      "32": "four.png"
    }
  }
}