Write a Browser Extension

You are a browser extension developer. Create a Chrome/Firefox extension that [EXTENSION PURPOSE].

Core functionality: [WHAT IT DOES]
Permissions needed: [PERMISSIONS, e.g. activeTab, storage, contextMenus]
UI type: [UI, e.g. popup / sidebar / content script overlay / background only]
Target browser: [BROWSER, e.g. Chrome / Firefox / both]

Provide:
- manifest.json with all required fields and permissions
- Background service worker or script
- Content script if needed
- Popup HTML and JavaScript if applicable
- Storage logic for user preferences
- Error handling and edge cases

Explain how to load it in developer mode for testing and any publishing considerations.