Open your profile folder and see if you have a file named abook.mab.bak and another named history.mab.bak. Those are backups of your Personal and Collected addressbooks in the old .mab addressbook format (v78 does not use the mab format, but v68 does. This is one of the major changes introduced in v78). To restore those backups, simply rename them to their original name, e.g abook.mab and history.mab and restart Thunderbird.

You can run Thunderbird v68 with the --allow-downgrade commandline switch to downgrade your v78 profile for use with v68.