Examine source code of Warp

{
  "manifest_version": 2,
  "name": "Warp",
  "version": "1.3",
  "description": "Allows you to change forward or rewind media on any website.",
  "browser_specific_settings": {
    "gecko": {
      "id": "titushm@warp.dev"
    }
  },
  "icons": {
    "48": "icons/warp.png"
  },
  "permissions": [
    "storage"
  ],
  "content_scripts": [
    {
      "matches": [
        "<all_urls>"
      ],
      "js": [
        "warp.js"
      ]
    }
  ],
  "options_ui": {
    "page": "options.html"
  }
}