Examine source code of Windowed - (No Windowed button)

{
  "name": "Windowed - floating Youtube/every website",
  "short_name": "Windowed",
  "description": "Changes fullscreen buttons to go into a popup. Works for every website that uses fullscreen, including Youtube, Vimeo, Netflix",
  "developer": {
    "name": "Michiel Dral",
    "url": "https://dral.eu/"
  },
  "version": "31",
  "manifest_version": 2,
  "permissions": [
    "storage",
    "tabs",
    "<all_urls>"
  ],
  "applications": {
    "gecko": {
      "id": "{469dbe5e-1769-4669-a269-b6113bb5cf6e}"
    }
  },
  "browser_action": {
    "default_popup": "Popup/Popup.html",
    "default_icon": {
      "32": "/Images/Icon_Windowed_Mono@1x.png"
    }
  },
  "content_scripts": [
    {
      "run_at": "document_start",
      "matches": [
        "<all_urls>"
      ],
      "js": [
        "Vendor/browser-polyfill.min.js",
        "Content.js"
      ],
      "all_frames": true
    }
  ],
  "background": {
    "scripts": [
      "Vendor/browser-polyfill.min.js",
      "Background/BackgroundEntry.js"
    ],
    "persistent": false
  },
  "web_accessible_resources": [
    "Images/*",
    "script_to_insert_directly_into_page.js"
  ],
  "icons": {
    "16": "Icons/Icon_16.png",
    "32": "Icons/Icon_32.png",
    "64": "Icons/Icon_64.png",
    "128": "Icons/Icon_128.png"
  }
}