Examine source code of Calculator Plus

{
  "manifest_version": 2,
  "name": "Calculator",
  "short_name": "Calculator",
  "description": "A simple calculator that remembers your last calculation. You can close it at any time, and reopen it without losing your work.",
  "version": "1.8.2",
  "offline_enabled": false,
  "icons": {
    "16": "css/icons/16x16.png",
    "24": "css/icons/24x24.png",
    "32": "css/icons/32x32.png",
    "48": "css/icons/48x48.png",
    "128": "css/icons/128x128.png"
  },
  "permissions": [],
  "browser_action": {
    "default_icon": {
      "19": "css/icons/19x19.png",
      "38": "css/icons/38x38.png"
    },
    "default_popup": "index.html",
    "default_title": "Calculator"
  },
  "browser_specific_settings": {
    "gecko": {
      "id": "{14f778ff-d440-4b9e-80fc-a9fdc0bc60e8}"
    }
  }
}