Examine source code of Ruler

{
  "manifest_version": 2,
  "name": "Ruler",
  "version": "1.2resigned1",
  "description": "Adds a ruler below your mouse to help read long lines. Fired by ctrl+click",
  "icons": {
    "48": "icons/ruler-48.png",
    "96": "icons/ruler-96.png"
  },
  "content_scripts": [
    {
      "matches": [
        "<all_urls>"
      ],
      "js": [
        "ruler.js"
      ]
    }
  ],
  "browser_specific_settings": {
    "gecko": {
      "id": "{bc094d57-4ea7-4216-b3ea-2cac72a5cc6b}"
    }
  }
}