whatisgithub

What is gecko-dev?

mozilla/gecko-dev — explained in plain English

Analysis updated 2026-07-03

★ 3,689JavaScriptAudience · developerComplexity · 5/5LicenseSetup · hard

In one sentence

A read-only Git mirror of the Firefox browser source code, provided so developers who prefer Git tooling can browse Mozilla's codebase, actual contributions go through Mercurial and Bugzilla.

Mindmap

mindmap
  root((gecko-dev))
    What it is
      Firefox source mirror
      Git copy of Mercurial repo
      Read only
    Use Cases
      Browse Firefox code
      Study browser internals
      Reference for contributors
    Contributing
      Bugzilla for bugs
      Mozilla docs site
      Matrix chat
    Tech Stack
      JavaScript
      C++
      Rust
Click or tap to explore — scroll the page freely

Code map

Detail Auto

An interactive map of this repo's files and how they connect — its source is parsed live in your browser. Click Visualize to build it.

filefunction / class

What do people build with it?

USE CASE 1

Browse and study how Firefox implements a specific browser feature using your preferred Git tools.

USE CASE 2

Clone the Firefox source to understand how a web API is handled at the browser engine level.

USE CASE 3

Use the mirror as a read reference while preparing a contribution to Mozilla's main Mercurial repository.

What is it built with?

JavaScriptC++RustPythonMercurial

How does it compare?

mozilla/gecko-devjgraph/drawio-mcpbigskysoftware/_hyperscript
Stars3,6893,6913,686
LanguageJavaScriptJavaScriptJavaScript
Setup difficultyhardeasyeasy
Complexity5/52/52/5
Audiencedeveloperdeveloperdeveloper

Figures from each repo's GitHub metadata at analysis time.

How do you get it running?

Difficulty · hard Time to first run · 1day+

This is a read-only mirror, building Firefox from source requires Mozilla's own build toolchain (mach) and takes significant disk space and compile time.

Mozilla Public License 2.0, you can use and modify the code, but changes to MPL-licensed files must be shared under the same license.

So what is it?

This repository is a read-only mirror of the Firefox browser source code. Mozilla, the non-profit organization behind Firefox, maintains the actual development history in a separate version control system called Mercurial, and this Git copy exists so that developers who prefer Git tooling can browse the code. The repository description notes it has been superseded by a newer mirror location. Firefox is a web browser focused on speed, reliability, and privacy. If you want to contribute to it, Mozilla points contributors to their documentation site and uses Bugzilla as the place to file and track bugs. There is also a community chat room on Matrix for people with questions about working on the browser source.

Copy-paste prompts

Prompt 1
I want to understand how Firefox handles the Fetch API under the hood. Which files in gecko-dev should I look at, and how is the request lifecycle structured?
Prompt 2
Show me where in the Firefox source code (gecko-dev) CSS layout and painting are handled, and explain the rough flow from CSS parsing to pixel output.
Prompt 3
I found a Firefox bug I want to fix. Walk me through the gecko-dev code to find where that browser behavior is implemented before I file a patch on Bugzilla.

Frequently asked questions

What is gecko-dev?

A read-only Git mirror of the Firefox browser source code, provided so developers who prefer Git tooling can browse Mozilla's codebase, actual contributions go through Mercurial and Bugzilla.

What language is gecko-dev written in?

Mainly JavaScript. The stack also includes JavaScript, C++, Rust.

What license does gecko-dev use?

Mozilla Public License 2.0, you can use and modify the code, but changes to MPL-licensed files must be shared under the same license.

How hard is gecko-dev to set up?

Setup difficulty is rated hard, with roughly 1day+ to a first successful run.

Who is gecko-dev for?

Mainly developer.

Open on GitHub → Ask about another repo

This repo across BitVibe Labs

Verify against the repo before relying on details.

What are similar repos?