Examine source code of twvolume

{
  "manifest_version": 3,
  "name": "twvolume",
  "version": "0.1",
  "description": "change volume using mouse wheel",
  "browser_specific_settings": {
    "gecko": {
      "id": "twvolume@latehours.net"
    }
  },
  "icons": {
    "16": "icons/icon_16.png",
    "48": "icons/icon_48.png",
    "128": "icons/icon_128.png"
  },
  "content_scripts": [
    {
      "matches": [
        "https://www.twitch.tv/*"
      ],
      "js": [
        "twvolume.js"
      ]
    }
  ],
  "host_permissions": [
    "https://www.twitch.tv/*"
  ]
}