What it is

Mercury is a web browser project in the Firefox ecosystem. It is a fork of Firefox that combines compiler optimizations with patches from Librewolf, Waterfox, Ghostery, and BetterFox. The project describes itself as the Firefox equivalent of Thorium, which is a Chromium fork. Mercury is licensed under MPL-2.0, and its repository topics include browser, privacy, optimization, and Firefox fork.

The concrete problem it solves is that stock Firefox does not include every optimization or patch that some users want in one build. Mercury gathers selected changes from other Firefox forks and named projects into a single source tree. It keeps Firefox as the base, so users get a browser fork rather than a new engine. The README also points to documentation for patches, bugs, building, and debugging, which makes the scope of the fork inspectable.

Key capabilities

• Mercury applies compiler optimizations to a Firefox fork, and its topics list avx, avx-instructions, sse3, and compiler-optimization.
• It includes patches from Librewolf, Waterfox, Ghostery, and BetterFox, as stated in the README.
• It is positioned around privacy, since privacy appears as a repository topic and several named projects contribute patches.
• It remains a Firefox browser fork, so it is built on the Firefox platform rather than on a separate rendering engine.
• It provides separate documentation for patches, bugs, building, and debugging, which supports source review and manual builds.
• It uses the MPL-2.0 license.

Who uses it and how

• Privacy-focused browser users may choose Mercury because privacy is a topic and because the project includes patches from Librewolf, Waterfox, Ghostery, and BetterFox.
• Advanced users may build the browser from source using BUILDING.md and DEBUGGING.md, then review PATCHES.md and BUGS.md before using the result.
• Firefox users may compare Mercury with stock Firefox as a fork that adds compiler optimizations and selected patches while keeping the Firefox base.
• Repository users may track open issues and documentation to understand maintenance status and known problems.

Getting started

The README does not mention Docker images, package managers, or a hosted service. It directs users to BUILDING.md and DEBUGGING.md for build instructions, and to PATCHES.md and BUGS.md for details about included changes and known problems.

When to use it — and when not to

Mercury is useful for users who want a Firefox fork that combines compiler optimizations with patches from named projects, and who are willing to build or inspect a source-based browser. It is less suitable for users who expect an official Firefox release, a packaged installer, or a low-maintenance hosted option, because the provided README only documents building, patches, bugs, and debugging. The project has 92 open issues and no listed contributor count, so users should review BUGS.md and repository state before relying on it for everyday use.