Does it appear in safe/troubleshoot mode (hold Shift when launching TB)? Do you have any signature-type add-ons? Is the jpg file still in the same location?

No it is not in safe mode; there are no add-ons or extensions; and the file has been in the same location for years. This only began after 10.01.2021. I logged into my mac and linux laptops and this does not happen.

Your picture shows the plain text message composer. An image signature only works with html messages. If plain text is your default, hold Shift when clicking Write or Reply to have the signature appear.

I disagree this was never an issue until I was asked to upgrade Thunderbird for Windows 10.

It works the same way in TB 78. Don't know if it was different in older versions.

Now I am on a debian laptop, using TB 78.14.0 and there is no problem. I will check the mac in a minute. Today I sent email and the signature appears. It was setup the same on all three laptops that I use.

Your last picture shows the signature in an html-format message, which we already know is possible. But it won't appear in a plain text message, in TB 78 or 91.

I am not understanding why you keep typing html-format message when this is not checked.

Look in the Composition & Addressing section (see picture). That is where the plain text or html format is chosen. If 'Compose messages in HTML format' is unchecked, the default format is plain text. The Write window menus are different for the two formats, e.g. plain text has no Insert menu or Formatting Bar.

I checked as suggested and that box was not checked. Interesting enough there are three other accounts I access mail using Thunderbird and the boxes are checked for all of them. However, when I clicked on write to create an email in the other accounts the signature did not show. After placing the check in the box for my work email address the signatures appeared in the other email addresses without a problem. Is this a bug for TB 91.2.1 (64-bit)?

Each account has its own default composition format, text or html, and signature settings. I haven't seen any effect of one account's settings on other accounts. But if you think there's a bug, submit a report to Bugzilla, as long as you can provide specific steps for others to reproduce the issue.

This only began after an update to 91.2.1. I am satisfied and for the next update I will check to observe if it happens again. Thanks for your patience and direction. Much appreciate!