How to Import your Address Book in Thunderbird?
Click “Text file (LDIF, .tab, .csv, .txt)” from the list of available options. Click “Next.”
Browse to the location where you saved your exported Outlook contacts and double-click on the CSV file.
Click the checkbox next to “First record contains field names.”
Click the “Move Up” and “Move Down” buttons to match the Outlook contact fields to those in Thunderbird.
Click “OK” and then click “Finish.”