First, check that autocomplete is enabled in Settings/Composition, and that your address books are populated.

Does the same thing happen in safe/troubleshoot mode (hold Option when launching TB)?

I could not find Autocomplete under Settings/Composition. When I searched for it in Settings, I was told there were no results for "autocomplete".

I have attached a snapshot of my Addressing settings.

I could find no way to check if my address books had anything in them, but if they don't, that is an even bigger error than not using them! I wasn't kidding about the swift and drastic nature of the change after I upgraded. It was working before and immediately not after the update. I wrote two messages in succession, one before and one after, and the names I was trying to put in I use literally every day and one of them in the message before the upgrade.

Your settings have autocomplete enabled, which is the default.

Open the Address Book tab (Ctrl+Shift+B) and see if the address books are intact, and that the email addresses are in the correct places. A TB upgrade shouldn't affect the address books unless the upgrade forces the creation of a new profile - in which case all account settings would be lost, too.

The two default address books in the profile folder are abook.sqlite (Personal Address Book) and Collected Addresses (history.sqlite). See if they are present in the active profile (Help/Troubleshooting Info, about:profiles, to view all profiles).

Repeat: Does the same thing happen in safe/troubleshoot mode (hold Option when launching TB)?

Sorry I did not respond yesterday. I was sick.

As with most keystroke shortcuts in TB for Mac, the keystroke just doesn't work. Nothing happens. I could not find a menu command for it.

I did not have to recreate my accounts, though, if that helps at all.

I'll try the troubleshoot mode (probably tomorrow, as I have to log into five accounts separately in TB) and let you know.

Thank you!

I tried it in safety mode. The autocomplete does work.

Now to just figure out which of my add-ons is causing the problem. . . .

I'll post which add-on, so folks know.

Thank you!

I disabled Auto Save Draft Folders (I wasn't using it anyway) and Mailbox Alert (which I think may have already been disabled as incompatible).

It's working now. I have a suspicion that just running it in Troubleshoot mode might have got it to connect to the address books, so if you're having the same problem, you might want to try just that before messing with add-ons.

Thank you, @sfhowes!