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.
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.
To view the source code of a specific version, you can also download it. Each product produced by Mozilla is archived there, together with its source code.