Mercury Browser is an optimized, privacy-focused web browser that is a fork of Mozilla Firefox. It incorporates compiler optimizations such as AVX, AES, LTO, and PGO to enhance performance and security. With features derived from projects like LibreWolf, Waterfox, and Ghostery, Mercury disables telemetry and debugging elements by default, ensuring a more private browsing experience. It also includes usability patches that bring back features like the classic top bar and supports unsigned extensions for added flexibility. Mercury is tailored for users prioritizing speed and privacy and is available for both Linux and Windows.