Examine source code of Block Sites i

{
  "manifest_version": 2,
  "name": "Block Sites",
  "version": "1.0",
  "description": "Blocks distracting websites.",
  "background": {
    "scripts": [
      "background.js"
    ]
  },
  "permissions": [
    "webRequest",
    "webRequestBlocking",
    "*://*/*"
  ],
  "web_accessible_resources": [
    "blocked.html"
  ],
  "browser_specific_settings": {
    "gecko": {
      "id": "{94d06c00-3a4d-4099-a36a-b5df9052c179}"
    }
  }
}