Examine source code of Popculator2

{
  "manifest_version": 2,
  "name": "Popculator2",
  "version": "1.2",
  "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"
  }
}