Having tried exporting to a CSV file and importing that - failed miserably - I exported to vCards and tried importing them. However Thunderbird imports them ONE AT A TIME!. They suggest an upgarde add-on but this downloaded a file which I didn't know how to open. The best idea was to open a Cmd window in the folder with all of the vcards and merge them by typing copy *.cvf merged.vcf. If you try to view the merged file, you get a succession of vcard popup windows so hold the Esc key down until they come to an end. Import the merged.vcf file using the import vcard option - it finishes with an error message but they will all have been imported perfectly.
