First, I have never attempted to modify headers, so my suggestions may be irrelevant. Your question interested me, so I poked around a bit: I noticed two addons that refer to headers: header tools like and header tools improved. You can also create custom headers with config editor - see https://www.lifewire.com/arbitrary-custom-heading-email-thunderbird-1173089

But I hope someone with experience jumps in to add comments.

David...

Thanks so much for replying but the old directions of how to find the section where the configuration editor is hiding are no longer applicable.

It is now under SETTINGS ---> GENERAL --> last item on page is Config Editor...

However, while using the command suggested by that other site did allow the entry of the code I mentioned in my post, unless I did something wrong because I did not see a "save" command, no change was made to my email headers as viewed in a received mail at a gmail address.

Other thoughts?

True, many web pages are still incorrect for some menu names. For the config editor, there is no save command. closing the editor is sufficient. You may need to reopen it to confirm that your entry is there. Otherwise, this is beyond my experience, as I've never modified headers.