Why is that, not all the merged e-mails contain the FirstName although the Outlook contact record does contain the FirstName?

(Applies to both EmailMerge Std and EmailMerge Pro)

This will only be used if Outlook recognizes the FirstName as being entered in the correct field. To check this, open the Contact record that has problems and click the Full Name button that will be on the left of where the actual contact's name is displayed.