Export and Import Contacts: Thunderbird

1. Open Mozilla Thunderbird on your computer and click on the Address Book item in the top menu bar.
2. In the Address Book window that opens, click on Collected Addresses (which contains all email addresses that you have sent to) or the name of the address book that you wish to export. This will cause the address book to be selected and the contacts from that address book will be shown on the right.
3. With the address book selected, click on Tools in the top menu bar.
4. From the options that show up, click on Export.
5. You will now be asked to select the file name and file type. Ensure that the file type is set to Comma Separated (System charset) ((.csv) or Comma Separated (UTF-8 charset) (.csv) or VCF.
6. Click Save.

Import

Now that you have saved the exported file,  import contacts from that file as explained here.