Source code for all Mozilla projects hosted in the Mercurial repositories can be searched and viewed online using Searchfox, a fast indexed search engine that runs on AWS.

Do not download the source code by crawling Searchfox; download a tarball (see instructions here) or use Mercurial  instead.

Searchfox indexes multiple branches and modules.

Some other Searchfox roots of interest are:

• "mozilla-central" contains current Firefox and Gecko development. Releases branch off of this repository. Also known as "Trunk" or "nightly".
• "comm-central" contains current Thunderbird, SeaMonkey and Calendar development. Also contains a mirror of mozilla-central. Releases branch off of this repository.

Mozilla Source Code Directory Structure has a short description of the code living in each source directory of the tree.