Examine source code of big rat

{
  "manifest_version": 2,
  "name": "big rat",
  "version": "1.0.0",
  "description": "replace every image on a website with a really cool big rat",
  "icons": {
    "48": "icons/bigrat.png"
  },
  "content_scripts": [
    {
      "matches": [
        "<all_urls>"
      ],
      "js": [
        "bigrat.js"
      ]
    }
  ],
  "browser_specific_settings": {
    "gecko": {
      "id": "{0d170162-ae25-4b9c-9b82-e856f7b721ac}"
    }
  }
}