Rated 5 out of 5 stars

MailMerge is improving each time ! However, since last time I used it with personnalized attachments, I moved to MacOS. I can't find the way to give the right path to the files and have it attached to the mails generated

This review is for a previous version of the add-on (9.1.0). 

You must select the files manually in the field "Attachments -> File" in the Mail Merge Dialog. Then specify the filename in the field "Attachments -> Attachments". See:
https://addons.thunderbird.net/thunderbird/addon/mail-merge/versions/9.0.0