Source code for all Mozilla projects hosted in the CVS repository can be searched and viewed online using MXR. It's a hacked variant of the original Linux Cross Reference tool.
MXR indexes multiple CVS branches and modules; the most interesting MXR root for a Mozilla contributor is the seamonkey root, which contains the latest source code for many Mozilla products and is updated frequently.
Some other MXR roots of interest are:
- "mozilla" root contains the trunk code, like "seamonkey", but indexes a bigger part of the repository and is updated less often.
- Mozilla 1.8 root contains Firefox, Thunderbird 2.0.0.x, and SeaMonkey 1.1.x
- Mozilla 1.8.0 root contains Firefox, Thunderbird 1.5.0.x, SeaMonkey 1.0.x
- Aviary 1.0.1 root Firefox and Thunderbird 1.0.0.x
- Mozilla 1.7 root contains Mozilla Suite 1.7.x
Mozilla Source Code Directory Structure has a short description of the code living in each source directory of the tree.