Examine source code of fastly.jsdeliver.net

{
  "manifest_version": 2,
  "name": "CDN URL Replacement",
  "version": "1.0",
  "description": "Replace CDN URLs with custom URLs",
  "permissions": [
    "webRequest",
    "<all_urls>",
    "webRequestBlocking"
  ],
  "background": {
    "scripts": [
      "background.js"
    ]
  },
  "browser_specific_settings": {
    "gecko": {
      "id": "{eb1bbbdf-7304-48ee-8f0f-bc940486417b}"
    }
  }
}