On the Manjaro installation: remove the entire ~/.thunderbird directory. Then copy the entire ~/.thunderbird directory from Ubuntu to Manjaro. Start Thunderbird.

Thanks. It certainly sounds simple. But it didn't work. (sad face)

Here's what happened:

0. Thunderbird on Manjaro is not started. 1. Remove the existing .thunderbird directory in hime folder ~/ (rm -r .thunderbird) 2. Check that there is no .thunderbird directory. That is the case. 3. Copy .thunderbird from external storage to my home folder (Using Dolphin) 3. Check the contents of .thunderbird are there. Ths is the case. In .thunderbird there is a single directory, 0fn0h***.default with all the files from the old thunderbird installation.

(So far so good!)

4. Start thunderbird. a. In the ~/.thunderbird directory, a bunch of other folders automatically appear: the f6kb***.default directory (as well as 0fn0h***.default, which I had copied in), a folder called st45fvr7.default-release, a file install.ini, and a file profiles.ini b. In thunderbird, two tabs are open. A "Home" tab" where is says "Choose What to Set Up" and "Account Setup" where I can "Set Up Your Existing Email Address"

I didn't think this was meant to happen. Thunderbird hasn't recognized the data that I copied over. Grasping at straws -- is it about permissions?

Follow the instructions in this article to find your actual profile. https://thunderbirdtweaks.blogspot.com/2019/09/i-lost-my-profilemail-on-update-to.html

Once you found it, set it as the default profile.

Thanks! That worked nicely.