Thunderbird Mailmerge send in plain-text, not html?

Question

Email

In Thunderbird, in the account settings, I unchecked the HTML checkbox, but Mail Merge, when previewing, shows that the text of the letter is formatted in html, i.e. shows all tags. I don't see how to switch mailmerge to plain text. in the mailing launch window (on the screenshot), there is no choice of html/plain-text, which was before.


How to make a letter in mailmerge without html, like plain text?