Examine source code of Focus Mode is

{
  "manifest_version": 2,
  "name": "Focus Mode",
  "version": "1.0",
  "description": "Block distracting websites to help you focus.",
  "permissions": [
    "tabs",
    "webRequest",
    "webRequestBlocking",
    "<all_urls>",
    "storage"
  ],
  "background": {
    "scripts": [
      "background.js"
    ]
  },
  "options_ui": {
    "page": "options.html",
    "open_in_tab": false
  },
  "browser_action": {
    "default_title": "Focus Mode",
    "default_icon": {
      "48": "icon.png"
    }
  },
  "browser_specific_settings": {
    "gecko": {
      "id": "{de8a82ae-364e-493b-a44e-6dbcf4d34b33}"
    }
  }
}