Examine source code of CardKingdomToCad

{
  "manifest_version": 2,
  "name": "CardKingdomToCad",
  "version": "1.1",
  "description": "Display cardkingdom.com prices as (CAD$) Canadian Dollars.",
  "icons": {
    "48": "icons/border-48.png"
  },
  "content_scripts": [
    {
      "matches": [
        "*://*.cardkingdom.com/mtg/*/singles*"
      ],
      "js": [
        "usdToCadPurchasing.js"
      ]
    },
    {
      "matches": [
        "*://*.cardkingdom.com/mtg/*"
      ],
      "js": [
        "usdToCadPurchasing.js"
      ]
    },
    {
      "matches": [
        "*://*.cardkingdom.com/catalog/*"
      ],
      "js": [
        "usdToCadPurchasing.js"
      ]
    }
  ],
  "permissions": [
    "<all_urls>"
  ],
  "browser_specific_settings": {
    "gecko": {
      "id": "{ab0bd058-733b-4b3a-8f35-fdadab17433c}"
    }
  }
}