The following screenshots cover the steps to create a mailing list in the safest possible way. These screenshots were taken in Thunderbird 1.5.

In your Personal Address Book, click New List

Enter a List Name and hit OK. If it doesn't work, choose a different name.

Double-click the new list.

Use autocomplete to add contacts.

When you're done, click OK.

You should now be able to send mail to everyone in your new list!

All contacts in a List must be in the Address Book which contains the List. Thunderbird will copy contacts over for you whenever possible, but this will lead to lots of duplicates if you have several Address Books.

If you're trying to make a new List and your OK button isn't working, there is something wrong with your new list. Thunderbird as of version 1.5 unfortunately doesn't tell you what the problem is. The most common problem is that the list name you chose already exists in another Address Book. You can have a contact with the same name in two Address Books, but you can't have a List with the same name in two places.

Never add a contact to a list before the contact has an email address associated with it. Thunderbird gets confused and bad things happen.