Thanks for the tip, Cindy. I am using Firefox 58.0.2. I had forgotten to clean the cache, so I did do that just now. When I logged back in here, FF gave me a little popup that says that the login box has an insecure connection (but still allowed me to log in), so I don't know if that's related.
I'll have to wait a couple of days to see if the Captcha message pops up again, and I'll report back to you.