Examine source code of Page Timer Tracker

{
  "manifest_version": 2,
  "name": "Page Timer Tracker",
  "version": "1.0",
  "description": "Track how much time you spend on each page for better time awareness.",
  "permissions": [
    "tabs",
    "storage"
  ],
  "background": {
    "scripts": [
      "background.js"
    ]
  },
  "content_scripts": [
    {
      "matches": [
        "<all_urls>"
      ],
      "js": [
        "content.js"
      ]
    }
  ],
  "icons": {
    "48": "icon.png",
    "128": "icon.png"
  },
  "applications": {
    "gecko": {
      "id": "pagetimertracker@example.com",
      "strict_min_version": "60.0"
    }
  }
}