Steps to reproduce:

Opened Thunderbird and allowed it to get mail.

Actual results:

Messages listed as usual, but when I click on one, it might show the content for the next message in the list. The correct message is lost. The next message still displays correctly,so it is now duplicated.

Expected results:

Messages should be displayed correctly as with all previous versions of Thunderbird.

2022-07-11_1317_mass-mark-TB102found

DUP -- see Bug 1778241. This is the compacting issue again, where the file is corrupted before the compact, which leaves misaligned headers, etc. REPAIR FOLDER fixes the misalignment at the cost of one or two messages which get lost. REPAIR should be run before compacting, so compacting should not be left for automatic (the new default).

Disable the new setting for automatic compacting at the bottom of the PREFERENCES (General) screen. When necessary to compact, manually run REPAIR FOLDER first to clean up latent problems in the folder(s), then manually compact the specific folder(s). Or repair all folders, then do the global compact.

The misaligned header pointers often cause partial messages to display -- so it might see a message with plain text without format code, or it could jump into html code at random -- varies per incident. Fix the pointers as above and the display should work. Beware, however, that since you already have some corruption, REPAIR FOLDER will fix what it can but may lose one or two messages -- "recovers" them as blanks. Then COMPACT works and shows those lost messages as blanks.