Examine source code of SmartUnitChanger

{
  "manifest_version": 2,
  "name": "Unit Converter",
  "version": "1.0",
  "description": "Automatically converts units (e.g., meters to feet, Celsius to Fahrenheit) on web pages.",
  "icons": {
    "48": "icons/icon.png"
  },
  "permissions": [
    "activeTab",
    "storage",
    "webRequest",
    "webRequestBlocking",
    "<all_urls>"
  ],
  "background": {
    "scripts": [
      "background.js"
    ],
    "persistent": false
  },
  "content_scripts": [
    {
      "matches": [
        "<all_urls>"
      ],
      "js": [
        "content.js"
      ]
    }
  ],
  "browser_action": {
    "default_popup": "popup.html",
    "default_icon": {
      "48": "icons/icon.png"
    }
  },
  "browser_specific_settings": {
    "gecko": {
      "id": "{939e0fae-594b-4eda-96bb-65056e69128e}"
    }
  }
}