Kotlin Firefox Extension manifest.json https://medium.com/@Cypressious/your-second-firefox-extension-in-kotlin-bafd91d87c41

manifest.json

{
  "manifest_version": 2,
  "name": "Beastify",
  "version": "1.0",
  "permissions": [
    "activeTab"
  ],

  "browser_action": {
    "default_icon": "icons/beasts-32.png",
    "default_title": "Beastify",
    "default_popup": "popup/choose_beast.html"
  },

  "web_accessible_resources": [
    "beasts/frog.jpg",
    "beasts/turtle.jpg",
    "beasts/snake.jpg"
  ]
}