This book is the definitive guide to building modern browser extensions, covering everything from fundamental concepts to advanced techniques. Developing a browser extension is more like building a mobile app than a traditional website. Extensions operate within a unique environment with their own security model, lifecycle, and APIs. This book demystifies the process, guiding you through every stage--from planning and development to publishing and maintaining your extension. You'll gain a deep understanding of how browser extensions function, their core architectural components, and best practices for structuring your code. The book also uncovers the nuances of extension development that many developers only discover through trial and error. By the end, you'll be equipped with the knowledge to confidently build, deploy, and scale a high-quality browser extension. You Will Learn The fundamental building blocks of browser extensions and how they interact with the browser How to avoid common pitfalls that can lead to security vulnerabilities, performance issues, and rejections from extension marketplaces To master the entire development lifecycle, from writing your first extension to publishing and maintaining it in the Chrome Web Store, Firefox Add-ons, and other repositories How to build browser extensions using modern tools, languages, and frameworks Best practices for extension networking, authentication, storage, and management New for This Edition: Coverage of the latest browser extension APIs, including the side panel, offscreen documents, user scripts, and vendor-specifc APIs like the Chrome Prompt API. Hands-on tutorials for building real-world extensions, including monetization strategies, AI assistants, password managers, ad blockers, and DevTools integrations The latest tooling and frameworks for browser extension development, such as Vite, Extension.js, and WXT