Thunderbird uses Junk mail controls and only creates a Junk folder if you set it up that way. If you are truly talking about a Spam folder you need to look elsewhere. Maybe your providers spam filter or any other spam software you are running.

Well - I am using "Spam Assassin". And I do have Thunderbird set up to create a junk file - though I've been puzzled by the loss of a junk icon at the top of the page that I once used to mark messages as junk. I have no idea now just how files get diverted into junk, since I don't have a way to mark them as such. Still, Mozilla has instructions about how to block a contact, but no hint about how to unblock it. That's what I'm looking for. The mail app on my Android cell announces that I've blocked messages from that person, but also gives no hint as to how to stop doing so. I'll google Spam Assassin to see if I can get any answer there.

Since thunderbird does not run on or affect the operation of Android devices, you need to look elsewhere as stated above.

If you marked the sender as junk at a minimum there should be a Junk icon beside the message in your inbox. If you setup junk controls and told Thunderbird to move junk I to a Junk folder that is where they would be. If there is a mention of spam or they are in a spam folder, it is not Thunderbird. Thunderbird avoids using the word spam so it is not blamed for actions like you describe. That does not work well.