Examine source code of Gmail POP3 Refresh Button

{
  "name": "Gmail POP3 Refresh Button",
  "description": "Refreshes all POP3 accounts when the POP3 button is clicked",
  "version": "0.1.6",
  "author": "StrangeMouse",
  "manifest_version": 3,
  "icons": {
    "16": "/images/gmail16.png",
    "32": "/images/gmail32.png",
    "48": "/images/gmail48.png",
    "128": "/images/gmail128.png"
  },
  "action": {
    "default_icon": {
      "16": "/images/gmail16.png",
      "32": "/images/gmail32.png",
      "48": "/images/gmail48.png",
      "128": "/images/gmail128.png"
    },
    "default_popup": "popup.html"
  },
  "content_scripts": [
    {
      "matches": [
        "*://mail.google.com/*"
      ],
      "js": [
        "content.js"
      ]
    }
  ],
  "permissions": [
    "storage"
  ],
  "browser_specific_settings": {
    "gecko": {
      "id": "GmailPOP3@strangmouse12345.org"
    }
  }
}