First, you will need to export your addressbook(s) as LDIF file(s).
To do that, open Netscape or Mozilla Mail and go into your Address Book. For each address book you want to export:
Select the addressbook you wish to export from the panel on the left.
From the File menu (in older versions of Netscape) or the Tools menu, choose Export.
Set the Save in: folder to be your My Documents folder.
Choose the Save as Type to be LDIF (*.ldi,*.ldif) and enter a file name for the address book, then choose Save.
If you have more than one addressbook, export each of those separately.
Once you've Exported your addressbooks, open Thunderbird.
In the tools menu, choose Import.
Make sure Address Books is selected.
Click Next.
Choose Text file (LDIF, .tab, .csv, .txt)
Click Next.
Set the Look in: to My Documents (or whatever directory you saved your LDIF file to)
Select the LDIF file you want to import.
Click Open.
When the addressbook has been imported, the message "Addresses successfully imported from Text File" will be displayed.
Click Finish.
Repeat for any other address books you wish to import.
-- jahb, last updated: July 30, 2004