1) Does this mean auto address completion when typing into the address box? Or you get an empty To: field when replying? It's been noticed that one at least one add-on breaks it. ChangeQuote, IIRC. There is, I am told, a new version on Kaosmos's website.

2) Uninstall Lightning, if installed, and re-install. You may have to go here for a compatible version.

Yes it means the auto address function linked to the address book does not work when typing into the address box!

As a follow-up. 1. I must now manually type in every address! 2. I tried to install: lightning-4.7b3-sm+tb-windows.xpi but still no luck! there is no latest version available.

Question: can I go back to the previous Thunderbird, what must I do without loosing anything?

Zenos said

2) Uninstall Lightning, if installed, and re-install. You may have to go here for a compatible version.

So TB 45 shipped with a bad version of Lightning included? ('Cause they updated together.) As for Lightning 4.7b3 or the later 4.7, when I tried, all I got was a message saying a required file could not be accessed.

This was all in addition to the TB toolbar being greyed out and unusable; buttons missing from the Customize menu; the Lightning Button not working with TB 45 (no catch for that incompatibility, either, btw).

And then, the forced change from From and Recipient to Correspondents requiring a lot of work to go back to what was preferred. I used to like TB because it didn't change my preferences during an update. I used to be offered choices, unlike other software. (I guess that's changed now.)

I've never had this much trouble with a TB update before (FF, yes, but never TB).

Epic fail, overall.