If you have a poor network connection that is preventing hg clone from completing, you may want to try downloading a bundle of the repository you're interested in. This is useful since a file download, unlike hg clone, can be resumed if the connection is interrupted. Once you have the bundle, staying up-to-date shouldn't take much time at all, if you keep up with it regularly.
Downloading a bundle
Up-to-date bundles of some of the repositories listed at https://hg.mozilla.org/ are available on a CDN at https://hg.cdn.mozilla.net/.
The gzip bundles are recommended. However, if your internet is slow, the bz2 bundles will be smaller (but will take several more minutes to apply).
Setting up the repository
Once you have downloaded the repository bundle, follow the steps below to recreate the repository locally based upon that bundle. Be sure to replace "mozilla-central" with the project you're working with as appropriate.
1. Initialize a new repository (in a directory called mozilla-central here):
mkdir mozilla-central hg init mozilla-central
2. Un-bundle the bundle file to that repository:
To use the below command in Windows, export the \path\to\hg and invoke the command from command prompt.
cd mozilla-central hg unbundle /path/to/your/bundle.hg
Get comfortable. Grab a coffee (or your favorite tasty beverage). Maybe a nap. This unbundling process is going to take quite a lot of time.
3. Add the following lines to the repository's config file so that Mercurial will automatically know where to pull changes from future updates. You can open the config file in your editor by running hg config --local
[paths] default = https://hg.mozilla.org/mozilla-central/
5. Update the repository to get all the changes since the bundle was created (this step also doubles as a check of the bundle integrity since if its contents are not exactly the same as what's in the official repository then the hg pull will fail):
hg pull
5. Check out a working copy from your new up to date repository:
hg update
You now have a clone of mozilla-central that is identical to one made via hg clone. You can adjust your build settings, or you can go straight ahead and build Firefox!