Locate your Contacts.csv file—by default saved on the Desktop—you just exported from Thunderbird.
Click Import.
A message will announce the successful completion of the Import process. Click OK.
Click on the imported contacts (in the picture below: Imported 12/13/11). Select More and choose Rename Group. When prompted by a pop-up window, type Thunderbird Contacts. Click OK.
You’re all set! Your contacts will appear in the middle column and will automatically be referenced when you compose your e-mail.