Must have add-on! Rated 5 out of 5 stars
Must to have add-on which makes Thunderbird as comprehensive and capable email program for sending out many customized letters at once.
In case you experience some trouble with umlauts in sent emails just try with different encoding mode in add-on options (UTF-8 or Windows-1252/ANSI). For example when you download CSV from Google Spreadsheet it's in UTF-8, so you should set Mail Merge encoding accordingly. When you open that file with MS Excel all umlauts are messed up and shown Ã¶Ã¶ etc. though. Don't let you bother from it. See how it looks with Notepad. And, most certainly, makes sure how it look in email by sending some test letters to your own address(es).
Of course there are odds and ends which can be better but definitely an add-on worth donation.