User Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.15; rv:109.0) Gecko/20100101 Firefox/109.0
Steps to reproduce:
It used to be possible to set the preferred mail format (text or html) on a per-contact basis. The field is now gone from the GUI, I don't know when it was removed. As of v91 the PreferMailFormat setting in the address book was apparently still recognized and respected. As of v102 the field (if present in abook.sqlite) is ignored.
Thunderbird 102.7.2 (64-bit)
macOS Monterey 12.6.3
There's no way to set the preferred email format per-contact. Mail format is determined by the global setting.
There should be a way to set the preferred email format per-contact.