Sidebery is a mature, feature-rich Firefox WebExtension that transforms the browser's sidebar into a highly customizable vertical tab manager. It provides tree-style tab management, integrated bookmarks and history panels, container-based tab isolation, session snapshots, search across tabs/bookmarks/history, context menus, keyboard shortcuts, drag-and-drop, custom CSS theming, and synchronization via Firefox Sync or Google Drive. The codebase is built with Vue 3, TypeScript, and Stylus, and follows a modular architecture with distinct background, sidebar, setup, and popup contexts communicating via a custom IPC framework. The source code reveals deep integration with Firefox's WebExtensions APIs and extensive use of reactive state patterns to maintain synchronized UI across multiple extension pages. The repository is actively maintained (latest commit August 2026) with frequent releases, a comprehensive issue tracker, and continuous integration via GitHub Actions. The code exhibits strong engineering practices: separation of foreground/background services, debounced storage operations, typed IPC actions, and a centralized utility module. However, some modules (notably `tabs.fg.ts` and `sidebar.fg.ts`) are large and tightly coupled, and there are minor signs of technical debt (deprecated settings, hardcoded error strings). Overall, the extension delivers on its promise of a powerful sidebar tab manager with an impressive degree of customization.