From MozillaZine Knowledge Base

You can use the Address Book Exporter 2.1.2 to export from the Mac OS X address book into a ".txt" file and then import into Thunderbird. Make sure to use the Yahoo option. This method is still not perfect but you can get it to work after a little bit of cut & pasting.

See also

• Importing an address list from a text file

• Online converter with downloadable PHP source code: vcfconvert
  • Select All contacts and Export to vCard. Will work with group cards, too.
  • Note when using source: rename index.1.php to index.php to get working - works OK, not complete conversion
  • Other converters available, search vcard convert ldif at google.