NONE of this worked for me yesterday, I had to reinstall 68.12.1 and because they CHANGED so much crap, my Address Book is Gone as well as the mail I sent, it only took 20 hours to download all the old mail from my server, which was Required before I could use it again. Updates are turned OFF.
I might just go back to Outlook if they continue to allow Children to write Code without ADULT SUPERVISION!!@!
For future updates or reinstall, copy your email off to a safe location.
Unfortunately Thunderbird does not make it easy such as Backup> Account but has you do searches on the internet on how to backup and import the email.
You gotta be a geek to do it !
Profiles - Where Thunderbird stores your messages and other user data
https://support.mozilla.org/en-US/kb/profiles-where-thunderbird-stores-user-data
Moving Thunderbird Data to a New Computer
https://support.mozilla.org/en-US/kb/moving-thunderbird-data-to-a-new-computer
The random yccklof6.default name is different for your install.
Open Thunderbird> Tools> Account Settings> Local Folders>
Local directory>
C:\Users\ Your name \AppData\Roaming\Thunderbird\Profiles\yccklof6.default\Mail
Your old email is there. You can copy and paste the folders into your new folder from the new install.
Inbox.sbd
Sent.sbd
Drafts.sbd
Templates.sbd