hello foss69llc, while you're in thunderbird, go to the menu > addons, search for "enigmail" and install it this way...

I was appoint to reply that I already tried this, but realized there was a step I had not taken before. So, letting you know success.

Thanks!

You got this error because the Extension is not compatible with well Firefox. Were you expecting Firefox to some how send the file to your Thunderbird install and install it there?

Either save the .xpi file to disk and install it in Thunderbird as per the How to install in Thunderbird instructions that popup when you click on Download Now button OR find it in the Addons manager search and install it that way as suggested above.

I think there is or was a Firefox extension that made it so the online hosted extensions would then install in Thunderbird and not Firefox.

I was actually following instructions provided on the site for Thunderbird, as well as Securityinabox, so would suggest the instructions at both sites misled. But solved now thanks to Philip's original reply, plus a bit of intuition on my part.

Thanks, though, for your suggestion. Just think that much of our earlier digital tech was more transparent and easier to use and modify.