Examine source code of TMIOSkinPreview

{
  "description": "Adds a Preview button to trackmania.io skin pages",
  "manifest_version": 2,
  "name": "TMIOSkinPreview",
  "version": "0.1",
  "homepage_url": "https://tm.byt3.dev/extension/firefox",
  "icons": {
    "48": "icons/border-48.png"
  },
  "permissions": [
    "https://trackmania.io/*",
    "webNavigation",
    "tabs",
    "scripting"
  ],
  "background": {
    "scripts": [
      "background.js"
    ]
  },
  "browser_specific_settings": {
    "gecko": {
      "id": "tm@byt3.dev"
    }
  }
}