Examine source code of Hercules

{
  "manifest_version": 2,
  "description": "This extensions tells you your time status in Zeus or Jira/Zeus percentage. This a port of Chrome Hercules extension.",
  "name": "CGM Hercules",
  "browser_action": {
    "default_icon": {
      "48": "hercules-48.png",
      "128": "hercules-128.png",
      "600": "hercules-600.png"
    },
    "default_popup": "popup.html",
    "default_title": "Hercules"
  },
  "background": {
    "scripts": [
      "scripts/extension.js"
    ]
  },
  "icons": {
    "48": "hercules-48.png",
    "128": "hercules-128.png",
    "600": "hercules-600.png"
  },
  "permissions": [
    "tabs",
    "activeTab",
    "<all_urls>"
  ],
  "version": "1.2"
}