Hey, firstly, you need to find out the name of the folder you need to download. I understand why you're downloading from CVS; the Mozilla Firefox/Thunderbird packages that are precompiled don't seem to be complete with the necessary files. Now, I'm not sure what the name of the folder branch is (the folder containing the source code you need to download), so basically you need to do the following.
Code:
cvs -d:pserver:anonymous@cvs-mirror.mozilla.org:/www login
At the login password prompt, type the password: anonymous
Finally, (this is after you find out the name of the repository) type this:
Code:
cvs -d:pserver:anonymous@cvs-mirror.mozilla.org:/www co <branchname>
Where <branchname> is replaced with, say, firefox (whatever the name of the package is.)
Hope this helps,
--Alex A