Using the Northwestern Directory as your Address Book

Northwestern's LDAP directory service contains the email addresses of everyone at Northwestern. You can tell Thunderbird to check that directory and complete addresses for you when you compose a new email.

Click on Tools | Account Settings.... The Account Settings dialog appears. Click on Composition & Addressing under your Northwestern email account, Use a different LDAP server: and the button Edit Directories....
The LDAP Directory Servers dialog appears. (Mine shows the NU LDAP I've already created, but your list will probably be empty.) Click on the Add button.
The Directory Server Properties dialog appears. Fill in the General tab as shown. Hostname is registry.northwestern.edu. Base DN is dc=northwestern,dc=edu. Port is 636. Use secure connection should be checked. Then click on the Advanced tab.
Fill in the Advanced tab as shown. Then click on OK.
Select NU LDAP from the list of LDAP directories, and click OK again.

Test your settings by creating a new email and typing a few characters of someone's name into the To field. Wait a few seconds and see if auto-completion presents a menu of names that match what you typed.

Note: for me, auto-completion of addresses does not work if I try to use NU LDAP as my global LDAP server, even though a bug report on this problem says it has been fixed.