I purged Thunderbird from my system, moved my .thunderbird folder aside,
installed Thunderbird, started Thunderbird, set up my e-mail account,
shut down Thunderbird, renamed my profile folder and adjusted
profiles.ini, and copied the address books and e-mail files and folders
from my old profile to my new profile.
I have configured Thunderbird not to synchronize messages from the
server to this computer.
I have created an Inbox subfolder on Local Folders, and created a
message filter to copy incoming messages from the server Inbox to the
local Inbox.
So, between a fresh install, a fresh profile, and changing my workflow
to avoid moving messages from IMAP folders to local folders, hopefully
Thunderbird will behave.
But, given that I sometimes see the toilet bowl mouse icon in Thunar, I
can only wonder if the problem might be Xfce or something deeper...
We'll see what happens.
Thanks for the help. :-)
David