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Please refer to Mercurial For Mozillians at ReadTheDocs for current best-practices around Mercurial, including many helpful extra tools and guidelines that make using Mercurial fast and easy.

Mercurial (also known as "hg"), is the distributed version control software used for the development of Firefox, Thunderbird, and the shared Gecko core. It replaced CVS after Mozilla 1.9 was branched.

hg is the Mercurial command-line tool, Hg being the chemical symbol for the element mercury.

Installation, configuration, and getting the source

See Installing Mercurial for installation and configuration tips.

See Getting Mozilla Source Code Using Mercurial for getting a tree to build.

Learning to use Mercurial

If you are new to Mercurial, you should start with the official guide.

Then, move on to Mercurial basics and Mercurial FAQ and the version control tool docs for Mozilla-centric Mercurial information.

Further reading

The Mercurial tag lists the Mercurial-related articles on MDN.

And on wiki.mozilla.org, these helpful pages: