Build Browser Extensions with Manifest V3

Browser Extension Builder is an expert skill for building Chrome, Firefox, and cross-browser extensions that people actually install and use daily, covering extension architecture, Manifest V3, content scripts, popup UIs, monetization strategies, and Chrome Web Store publishing. Its standard project structure separates manifest.json, a popup/ UI, a content/ script that runs on web pages, a background/service-worker.js for background logic, an options/ settings page, and icon assets. The Manifest V3 template wires these together:

{
  "manifest_version": 3,
  "name": "My Extension",
  "version": "1.0.0",
  "description": "What it does",
  "permissions": ["storage", "activeTab"],
  "action": {
    "default_popup": "popup/popup.html",
    "default_icon": {
      "16": "icons/icon16.png",
      "48": "icons/icon48.png",
      "128": "icons/icon128.png"
    }
  },
  "content_scripts": [{
    "matches": ["<all_urls>"],
    "js": ["content/content.js"]
  }],
  "background": {
    "service_worker": "background/service-worker.js"
  },
  "options_page": "options/options.html"
}

The popup, background service worker, and content script communicate through chrome.runtime messaging and shared chrome.storage.

Use this skill when starting a new browser extension, writing content scripts that read or modify page content, persisting settings via chrome.storage.local (5MB limit) or chrome.storage.sync (100KB total, 8KB per item), planning extension monetization, or preparing a Chrome Web Store submission. Since extensions can't call payment processors like Stripe directly, monetization routes through your own backend - the popup opens your website with a user ID, and after payment your API is polled to sync a premium-status flag back into extension storage; Chrome itself discontinued built-in payments. It ships validation checks with severity and fix actions: using deprecated Manifest V2 (High - migrate to V3 with a service worker), requesting excessive permissions (High - use scoped host_permissions and optional_permissions), missing chrome.runtime.lastError checks (Medium), hardcoded URLs (Medium - move to storage or manifest config), and missing store-required icon sizes (Low). Don't use it for the extension's popup UI framework itself (routed to a frontend skill), a purely personal one-off tool, or an AI-powered extension's model integration, all of which are delegated to other skills.

Input is the extension's intended functionality and target browsers; output is a Manifest V3 project structure, content-script code for reading or injecting UI onto pages, an async storage wrapper around the callback-based chrome.storage API, and a monetization flow (freemium, one-time, subscription, donation, or affiliate) wired through an external payment backend.

Developers building Chrome, Firefox, or cross-browser extensions - from popup tools to AI browser assistants - who want proven Manifest V3 architecture, content-script patterns, and store-safe monetization rather than assembling extension structure from scratch.