To export your contacts from Mozilla Thunderbird, use the Address Book: to open Thunderbird's address book, go to Tools > Address Book from Thunderbird's main window.
Ctrl+2 Open Thunderbird's address book
As we saw in the Thunderbird Address Book tutorial, all your contacts are stored in one or more address books, accessible from the Address Book tool.
Thunderbird supports multiple address books: the export address book functionality will export the contacts from the selected address book. To export contacts from other address books, repeat the procedure outlined below for each address book.
If you are planning on exporting your address books' contacts for another Thunderbird installation, look instead at the Backup and Restore tutorial, which shows you how to find your Thunderbird profile data.
After having selected the address book you would like to export, go (from Thunderbird's address book window) to Tools > Export.
The Export Address Book dialog will open, and let you pick a file name, a location and a file format for your exported contacts.
Thunderbird lets you export contacts from your address book in three formats: LDIF, for LDAP directories, CSV (Comma-Separated Values), the most compatible used format (where contacts are listed on each line, and contact information is separated by commas), or Tab Delimited.
The CSV format enables you for example, to import the contacts you exported from Thunderbird's address book into Microsoft Outlook or Outlook Express.
When exporting address book contacts as CSV files, Thunderbird includes only values, and no header information.
Microsoft Outlook and Outlook Express, on the other hand, import and export CSV files using headers to import contacts.
You will still be able to import Thunderbird exported address books, but you will have to map custom fields, i.e. confirm to the importing email client which comma-separated ("column") value corresponds to which field type (first name, last name, primary email address, etc.).
Surprisingly, Microsoft Outlook does not natively import LDIF address book, but Outlook Express does. The easiest way to import LDIF contacts into Outlook is through Outlook Express.
Advantages of using LDIF as address book format are LDIF's "open-standardness"; the predictable format allows third-party application to either directly import it, or use an easy to write utility to convert from LDIF to a proprietary format.