Examine source code of Limit Your Tabs

{
  "manifest_version": 2,
  "name": "Tab Limiter",
  "version": "1.0.0",
  "description": "Limit the number of tabs you can open in any browser window",
  "permissions": [
    "tabs",
    "storage",
    "notifications"
  ],
  "background": {
    "scripts": [
      "background.js"
    ]
  },
  "browser_action": {
    "default_popup": "popup/popup.html",
    "default_icon": {
      "16": "icons/icon16.png",
      "48": "icons/icon48.png",
      "128": "icons/icon128.png"
    }
  },
  "icons": {
    "16": "icons/icon16.png",
    "48": "icons/icon48.png",
    "128": "icons/icon128.png"
  },
  "browser_specific_settings": {
    "gecko": {
      "id": "tab-limiter-extension@example.com",
      "data_collection_permissions": {
        "required": [
          "none"
        ]
      }
    }
  }
}