Examine source code of Protonmail Avatars

{
  "manifest_version": 2,
  "name": "Protonmail Avatars",
  "version": "1.12.9",
  "description": "Unofficial plugin. Displays avatar icons for senders in the Protonmail interface",
  "homepage_url": "https://mail.protonmail.com/",
  "author": "C.Oliff",
  "icons": {
    "16": "icons/icon16.png",
    "48": "icons/icon48.png",
    "128": "icons/icon128.png"
  },
  "browser_action": {
    "default_icon": "icons/icon48.png",
    "default_name": "Protonmail Avatars"
  },
  "content_scripts": [
    {
      "css": [
        "css/extra.css",
        "css/fa.css"
      ],
      "matches": [
        "https://mail.protonmail.com/*"
      ]
    }
  ],
  "content_security_policy": "script-src 'self'; object-src 'self'",
  "permissions": [
    "activeTab",
    "https://mail.protonmail.com/*"
  ],
  "optional_permissions": [
    "https://*/*"
  ],
  "background": {
    "scripts": [
      "background.js"
    ],
    "persistent": true
  }
}