Mail merge is one of those things that we would all like to do from time to time, send a personalized email to a list. Most people are familiar with mail merge in Microsoft Word, but due to a quirk in the way Word implements Simple MAPI and a deficiency in Thunderbird, it just does not work.
The answer is simply,
either use another word processor like LibreOffice Libre Office sends it's own mail merges without any reference to a mail program at all. It also quite happily uses the Thunderbird address book to address the mail and is my prefered method of mail merging..
Install the Mail Merge add-on for Thunderbird and do it all from Thunderbird,