Examine source code of encrypted email sender for Gmail

{
  "manifest_version": 2,
  "name": "Secure HTML Mail Sender for Gmail",
  "version": "1.6",
  "description": "Create encrypted HTML emails and insert them into Gmail composer",
  "permissions": [
    "activeTab",
    "tabs",
    "storage",
    "https://mail.google.com/*"
  ],
  "browser_action": {
    "default_popup": "popup.html",
    "default_icon": {
      "16": "icons/icon16.png",
      "32": "icons/icon32.png",
      "48": "icons/icon48.png",
      "128": "icons/icon128.png"
    }
  },
  "icons": {
    "16": "icons/icon16.png",
    "32": "icons/icon32.png",
    "48": "icons/icon48.png",
    "128": "icons/icon128.png"
  },
  "background": {
    "scripts": [
      "background.js"
    ],
    "persistent": false
  },
  "content_scripts": [
    {
      "matches": [
        "https://mail.google.com/*"
      ],
      "all_frames": true,
      "js": [
        "content.js"
      ]
    }
  ],
  "browser_specific_settings": {
    "gecko": {
      "id": "secure-html-mail-sender@local",
      "strict_min_version": "109.0",
      "data_collection_permissions": {
        "required": [
          "none"
        ]
      }
    }
  }
}