This repository is the source code for the Zen Browser, a Firefox-based desktop browser focused on productivity and customization. The codebase is a heavily modified Firefox fork that adds custom features such as workspaces (called Spaces), visual page boosts (duotone color filters), split-view tabs, glance previews, tab folders, compact mode, live folders, and a device sync system for workspace/folder/tab state. The architecture is a monolithic browser application built on top of the Gecko engine, with custom C++ XPCOM components for platform-specific functionality (e.g., native window dragging, mouse tracking for compact mode, color filter backend using Oklab color space) and JavaScript modules for the UI layer. The project includes an extensive test suite covering session restore, sync engines, shell integration, and pixel-level boost validation. It relies on upstream Firefox source code (patched via .patch files) and external services like Firefox Sync and Remote Settings. The codebase demonstrates strong engineering with well-organized custom modules, reusable test infrastructure, and a focus on state persistence and synchronization, though it carries the inherent complexity and coupling of a browser fork.