Examine source code of Preview Mode

{
  "manifest_version": 2,
  "name": "Preview Mode Opener",
  "version": "1.0",
  "description": "Opens the current site with /preview?lang= appended to the URL.",
  "permissions": [
    "activeTab",
    "tabs"
  ],
  "background": {
    "scripts": [
      "background.js"
    ],
    "persistent": false
  },
  "browser_action": {
    "default_icon": {
      "16": "icons/icon16.png",
      "48": "icons/icon48.png",
      "128": "icons/icon128.png"
    },
    "default_title": "Open in Preview Mode"
  },
  "icons": {
    "16": "icons/icon16.png",
    "48": "icons/icon48.png",
    "128": "icons/icon128.png"
  },
  "browser_specific_settings": {
    "gecko": {
      "id": "{cc8449bb-7e7c-4fb0-985c-70b181076d9f}"
    }
  }
}