Examine source code of On-Page Timer

{
  "manifest_version": 2,
  "name": "On-Page Timer",
  "version": "1.1",
  "description": "Track how long you've spent on the current page.",
  "applications": {
    "gecko": {
      "id": "{53095bd7-4e61-4eb4-b827-2027e7d730b7}",
      "strict_min_version": "112.0"
    }
  },
  "permissions": [
    "tabs",
    "storage"
  ],
  "background": {
    "scripts": [
      "background.js"
    ]
  },
  "browser_action": {
    "default_popup": "popup.html",
    "default_title": "Page Timer",
    "default_icon": {
      "64": "logo64.png",
      "128": "logo128.png"
    }
  }
}