After posting this question, I find that Thunderbird has suddenly righted itsself, and the problem has vanished!

It started again, so I posted a new question because I forgot about having posted this one!

I have no idea what During a busy time copying data from emails to an online form (Thunderbird and Firefox) really means, but it is quite probable that you have consumed your daily bandwidth allowance from you mail provider and they have shut you down until it resets.

I confused the question by saying what I was doing when I noticed the problem. The issue is that, apparently randomly, Thunderbird stops showing new massages emboldened and stops adding to the displayed number of unread messages. Yesterday, I did a Windows 10 update, but the problem continues, as it has for several months. I am not aware of any email limit with my domain's host. Indeed, emails are not lost, it is simply that some of them are not emboldened. The glitch seems to be in Thunderbird running under Win 10 on an old HP Elitebook.