But if I: Click write, type first recipient in the first To: box. Press Enter. I expect that the focus should jump to the next line, allowing me to add a new To: <recipient>. And that is what happens on my laptop.

This is what happens here too. We're talking about TB68, do we?

However, on my desktop, when I press Enter, the cursor just sits at the end of the first recipient, no matter how many times I press Enter.

Not sure what's causing this. Is there any difference when you click into that line? I've noticed when completing the recipient's address with a tab after starting to type, then there is no cursor, and one can press Enter as many times as you want, nothing happens.

Does the same behavior occur in safe mode? https://support.mozilla.org/kb/safe-mode-thunderbird

TB 68.11.0 (32-bit).

When I restart in safe mode, Enter functions as it should (type first recipient, press enter, cursor auto-advances to the next recipient.

I had two add-ons. Mail-Merge was one, when I disabled, it did not fix the problem.

The other one was Nostalgy. But I found that I had TWO copies of it. One by opto (1.0.16a of Dec 23, 2019), and one by the original author Alain Frisch (0.3 of Feb 19, 2020). I removed the Opto version and everything works fine now.

Thanks for the pointer to safe mode. I had forgotten about it.

On Thunderbird 78.4.3. Now when I go to write a new email I no longer get a line for each email recipient; they drag on and on in the same field and the layout of the recipients is very difficult to work with. No longer able to view each recipient on it own line or field. Since they are all listed in one field run together I can no longer change an individual recipient from/to TO: and CC: and BCC: at will.

Please provide a fix so the recipient list works the way it used to work.