Build document in wiki.mozilla.org is too old. I explain enlistment of Thunderbird 3.
checkout comm-central repository
Thuderbird 3 / Seamonkey 2 repository is moved from cvs.mozilla.org to hg.mozilla.org.
$ hg clone http://hg.mozilla.org/comm-central/
checkout other project
Thunderbird 3 depends on mozilla-central and other project such as LDAP C-SDK. So you need checkout other projects
$ cd comm-central $ python client.py checkout
Good luck!