Browser extensions are small applications that run inside Chrome, Firefox, Safari, and Edge to extend or modify how users interact with web pages. Manifest V3 is now the only supported manifest in Chrome (since 2024); Firefox supports both V2 and V3. The architecture has three pieces: a service worker (background logic), content scripts (injected into pages), and an optional popup or side panel UI.
The market for paid extensions is small but real — productivity tools, writing assistants, ad blockers, and developer utilities make up most of the successful paid extensions. Most courses cover the same ground: messaging between contexts, storage APIs, permissions model, and the Chrome Web Store / Firefox AMO publishing flow.