It is an email standard. Thunderbird conforms to standards. https://www.lifewire.com/how-to-use-the-standard-email-signature-delimiter-1171204

This thread tells you how to turn it off. https://support.mozilla.org/en-US/questions/1019583

See post below.

I noticed after I made the toggle adjustment, the change was automatic and I didn't need to restart Thunderbird. But when I closed the program for the night, I noticed Thunderbird did an automatic update. This surprised me as I don't usually allow automatic updates.

This morning when I opened Compose, I found TWO sets of dashes above my signature. The mail sig suppress line toggle had been turned back to false, and when I toggled it to true again, I was left with ONE set of dashes. So it seems like I can't turn off those effing dashes.

Wtf. Are these really important to someone?

This time after attempting the toggle and closing out the program Thunderbird did another update.

I'm not sure if this is a prank. My anti-malware was firing up yesterday as well. If so, it's kind of creepy.

Yes, they are important to me.

If you use an add-on to choose between multiple signatures (don't you use different signatures for different correspondents?) then the dash dash space delimiter is essential for it to work.

They are also a de facto standard in newsgroups and they are important to me here too.

Thanks for the explanation. I still prefer to remove the dashes. I only use one signature on one account and I don't need to the ability to switch to other signatures.

Do you have any insight into why the above-offered fix suddenly no longer works, as of today?

I am afraid not. As you can see, I want and do use the dashes, so I have little experience with the switch to turn them off. I'm baffled as to what's gone wrong for you.