When upgrading a 64bit version of Thunderbird version 60 to version 68, the existing profile wasn't recognized and a new profile was created.
Note: If your profile is still not recognized, select it by visiting about:profiles in the Troubleshooting Information.
When upgrading from Thunderbird version 60 to version 68, add-ons are not automatically updated during the upgrade process. They will however be updated during the add-on update check. It is of course possible to reinstall combatible add-on via the Add-ons Manager or via addons.thunderbird.net.
LDAP lookup not working when SSL is enabled. Workaround: Disable SSL or switch off option "Query OSCP responder servers" in the certificate settings in advanced options.