Examine source code of Page Timer

{
  "manifest_version": 2,
  "name": "Page Timer",
  "version": "2.2resigned1",
  "description": "Simple seconds counter fires on document ready",
  "icons": {
    "48": "icons/page_timer-48.png",
    "96": "icons/page_timer-48_2x.png"
  },
  "content_scripts": [
    {
      "matches": [
        "<all_urls>"
      ],
      "js": [
        "page_timer.js"
      ]
    }
  ],
  "browser_specific_settings": {
    "gecko": {
      "id": "{8a9f2ed4-9380-497d-96da-d6d402759ff2}"
    }
  }
}