After having a look at the code of the thunderbird extension, i commented out some code in components\mpe.js (around line 293) to make it look like:
out += 'CARID:' + EncodeQP(Card.phoneticLastName) + ';LEDIT:' + Card.lastModifiedDate + /*AddV('FNAME', Card.firstName)*/
/*AddV('FNAME', Card ...