The folder pane is broken after upgrading from TB 91.7.0 to TB 91.8.0.
The folder pane is still broken after fresh installing TB 91.8.0.
There is a workaround in the thread. Maybe helpful to someone else.
Phoenity Icons 3.4
Thunderbird Portable 91.8.0 with 2 Google IMAP accounts
Windows 10 Pro 20H2 32-bit