Examine source code of Popculator

{
  "manifest_version": 2,
  "name": "Popculator",
  "version": "1.1",
  "description": "A simple Calculator Addon for Firefox",
  "icons": {
    "48": "48.png",
    "96": "96.png"
  },
  "permissions": [
    "activeTab"
  ],
  "browser_action": {
    "default_icon": "Calculator.png",
    "default_title": "Popculator",
    "default_popup": "index.html"
  },
  "content_scripts": [
    {
      "matches": [
        "<all_urls>"
      ],
      "js": [
        "script.js"
      ]
    }
  ]
}