Thunderbird upgrade to 45.0 does a database conversion !
 

Slack64 Current Multilib.
The Fri Apr 15 20:37:37 UTC 2016 Thunderbird upgrade to 45.0 does a database conversion: the old "From" and "Recipient" columns are replaced by a new "Correspondents" one. When accessing my converted mail folders with a previous (<= 38 ?) Thunderbird version the "From" and "Recipient" columns are gone ...
As per https://bugzilla.mozilla.org/show_bug.cgi?id=1152706 this conversion can be disabled, but for the moment I am "holding" the upgrade until all my Thunderbird versions can be upgraded.
:D

here doesn't look like the mail database is converted at all: the original mails are still stored in your ~/.thunderbird directory in the mbox format.
maybe you are talking about the column view in the GUI but you can customize it as you like (even showing again the From and Recipient fields) using the dedicated button (just above the messages list, on the right side).

You are right: it is a GUI thing.
The "45" GUI has a "tendency" to [re-]select the "Correspondents" column (and deselect the "From" and "Recipients" columns). And the "38" GUI then opens with the "From" and "Recipients" columns deselected, while there is no "Correspondents" column ...
So things are less dramatic than I first thought.
Thks, Dick
:D