Examine source code of SLEEP!

{
  "manifest_version": 2,
  "name": "SLEEP!",
  "version": "1.1",
  "description": "Set a sleep interval where you are blocked from all sites.",
  "icons": {
    "48": "icons/sleep.png"
  },
  "browser_action": {
    "default_icon": "icons/sleep.png",
    "default_title": "SLEEP!",
    "default_popup": "popup/input.html"
  },
  "permissions": [
    "<all_urls>",
    "activeTab",
    "tabs",
    "storage"
  ],
  "content_scripts": [
    {
      "matches": [
        "<all_urls>"
      ],
      "js": [
        "blocker.js"
      ]
    }
  ],
  "browser_specific_settings": {
    "gecko": {
      "id": "{9a9d2dad-c53b-4aee-b9df-cfd2c1728625}"
    }
  }
}