lets start5 with the important part. Thunderbird 68 was the last to use abook.mab and hisotory.mab. In V78 they were converted to sqlite files and are stored in an SQlite database now.

But I can see no reason why they would disappear with the update as there has been no major changes to the storage format in the current version 91 (which is still not offering upgrades, so I must assume you chose to install an updated version you downloaded.)

Did you get asked to setup your mail accounts again after the update?

I had not updated Thunderbird in some time, so I downloaded the "latest" setup file and ran it. The install did not ask me to set up mail accounts again. All account information, mailboxes, other preferences, etc, were unchanged. What appears to be the profile in use (in appdata/local) has several .sqlite files.

Hope this helps.