Examine source code of Focus Tab

{
  "manifest_version": 2,
  "name": "Focus Tab",
  "version": "1.0",
  "description": "Dim all other tabs except the current one to help you focus.",
  "permissions": [
    "tabs",
    "activeTab"
  ],
  "background": {
    "scripts": [
      "background.js"
    ],
    "persistent": false
  },
  "browser_action": {
    "default_icon": {
      "48": "icons/icon-48.png",
      "96": "icons/icon-96.png"
    },
    "default_title": "Focus Tab"
  },
  "icons": {
    "48": "icons/icon-48.png",
    "96": "icons/icon-96.png"
  },
  "browser_specific_settings": {
    "gecko": {
      "id": "{a6259619-d0d7-45df-8973-454c8b144359}"
    }
  }
}