The mozilla.org CVS repository can be downloaded via rsync. This is useful for faster diffs or for attempting to import it into another version control system.
Here's how to do it:
rsync -az --delete --stats cvs-mirror.mozilla.org::mozilla ~/cvs-mirror
This will mirror the CVS repository into ~/cvs-mirror. Optionally, the --progress option will show the progress for each file, and a percentage of the total files that are transferred.
At the time of this writing, this will take 2.8 GB of disk space.
Note that for using this copy, the NoLocks line in CVSROOT/config will need to be commented out, as it requires a modified CVS binary.
References
Thanks also to bz for telling me the exact commands that I need