I followed the directions at:
How to update Thunderbird 78 to 91 on Ubuntu 20.04?
to install Thunderbird 91 in Kubuntu 20.04:
Remove the apt version, and install the Thunderbird 91 snap, by choosing the "candidate" channel instead of "stable". This is a very safe way to upgrade, and recommended.
I then tried to point Thunderbird 91 to my existing profile on a second drive by starting Thunderbird from a terminal with the -p switch.
$ thunderbird -p
The error message:
Could not read the contents of /mount Error opening directory /mnt: Permission denied
and in the terminal:
GTK_FILE_CHOOSER_ACTION_SELECT_FOLDER is not supported by GTKFileChooserNativePortal because portal is too old
Unfortunately, moving the profile folder to a different drive isn't an option for me.