Examine source code of base64 decoder

{
  "manifest_version": 2,
  "name": "base64 decoder",
  "description": "select some text, and base64 decode it.",
  "version": "1.1resigned1",
  "applications": {
    "gecko": {
      "strict_min_version": "48.0",
      "id": "{b20e4f00-ab03-4a88-90e7-4f6b6232c5a9}"
    }
  },
  "background": {
    "scripts": [
      "background.js"
    ]
  },
  "content_scripts": [
    {
      "matches": [
        "<all_urls>"
      ],
      "js": [
        "base64coder.js"
      ]
    }
  ],
  "permissions": [
    "activeTab",
    "tabs",
    "contextMenus",
    "storage",
    "<all_urls>"
  ],
  "icons": {
    "16": "icons/page-16.png",
    "32": "icons/page-32.png",
    "48": "icons/page-48.png"
  }
}