git-cinnabar wiki explains how to set up a repository for Firefox development in Mozilla: A git workflow for Gecko development.
The first step of that tutorial,
git clone hg::https://hg.mozilla.org/mozilla-unified gecko may fail if your internet connection is unreliable.
If you have trouble with that step, an alternative is to use Mercurial Bundles once, to initialize the repository.
This tutorial walks you through the creation of a
gecko repository for Firefox/Gecko development, initialized using Mercurial Bundles.
Commands are shown on the lines starting with
$, and the output is shown for reference. You can copy and paste the command in the shell.
You need at least 10 GB of free disk space for this. On October 14th, 2020: 1.2GB for the bundle, 7GB for the repository, and a bit more for dependencies and creating an [ar