Examine source code of Overlay Clock

{
  "manifest_version": 2,
  "name": "Overlay Clock",
  "version": "1.0",
  "description": "Shows digital clock as overlay rectangle to Firefox browser window",
  "icons": {
    "48": "icons/overlayclock-48.png",
    "96": "icons/overlayclock-96.png"
  },
  "content_scripts": [
    {
      "matches": [
        "<all_urls>"
      ],
      "js": [
        "overlayclock.js"
      ]
    }
  ]
}