Examine source code of Fetch Response Modifier

{
  "manifest_version": 2,
  "name": "Fetch Response Modifier",
  "version": "2.1",
  "description": "Modifies the response from fetch requests to specific URLs.",
  "icons": {
    "128": "icon.png"
  },
  "permissions": [
    "webRequest",
    "webRequestBlocking",
    "https://www.duolingo.com/*"
  ],
  "content_scripts": [
    {
      "matches": [
        "https://www.duolingo.com/*"
      ],
      "js": [
        "content.js"
      ],
      "run_at": "document_start"
    }
  ],
  "background": {
    "scripts": [
      "background.js"
    ]
  },
  "browser_specific_settings": {
    "gecko": {
      "strict_min_version": "130.0",
      "id": "{9337f04a-dde8-40ad-872d-36c7de2a3c91}"
    }
  }
}