"Recents" is just a view that is either turned ON or OFF. It cannot be deleted. Use the View menu > Folders > Recents to toggle it back ON.

Unfortunately, it didn't work. I have both All and Recents checked on the View>Folders menu. what happens now is that when I want to file a message and click on File, the Recents folder is there, but as soon as I mouse down to select it, it greys out. Then the next time I repeat the process, after I click on File, the Recents folder on the drop-down menu is greyed out before I even mouse down there.. I thought that having All selected as well might have screwed things up, so I tried unselecting it, and that didn't help.

When I start Thunderbird from scratch, instead of seeing all my folders in the left-hand column, I *just* see the recent mail servers I have contacted. I wonder if we are talking about the same "recents" What I was referring to is the Recents folder that appears in the dropdown from the File icon, and which used to contain the file folders I had filed messages to recently. I've noticed that the folders in this Recents folder used to disappear after I had not fled anything there for a time (30 days?).

I now get what you meant by Recents. You customized the Mail Toolbar and placed the File button there, which literally moves the selected message to a target folder of your choice. I don't know how you tried to edit that historical list of folders you recently used as targets for the File action, but that Recent folder should reappear once you've performed the file action by choosing an existing target folder.