Examine source code of Beastify2

{
  "description": "Adds a browser action icon to the toolbar. Click the button to choose a beast. The active tab's body content is then replaced with a picture of the chosen beast. See https://developer.mozilla.org/en-US/Add-ons/WebExtensions/Examples#beastify",
  "manifest_version": 2,
  "name": "Beastify",
  "version": "1.1resigned1",
  "homepage_url": "https://github.com/mdn/webextensions-examples/tree/master/beastify",
  "icons": {
    "48": "icons/beasts-48.png"
  },
  "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"
  ],
  "browser_specific_settings": {
    "gecko": {
      "id": "{1498b061-f7fc-47d7-88e6-3b9d8a7fb1b9}"
    }
  }
}