For Windows users, the file path is something like this: [C:\Users\username\AppData\Roaming\Thunderbird\Profiles\]
   For macOS it's [Macintosh HD/Users/username/Library/Thunderbird/Profiles/***default.] 

   Converted 'abook.mab' to 'abook.mab.bak'
   Generated a new 'abook.sqlite file',
   Generated a new 'abook-2.sqlite' file, (but this abook-2.sqlite file is empty).

After updating to TB 78, I lost the contents of multiple address books. I could see contents of the default address books - Personal Address Book (abook.sqlite) and 'Collected Addresses' (history.sqlite). I was only missing the contents from all the address books that I created. These are the step by step instructions I used to recover theses address books. You need to locate and open the "Profile" folder. You can do this from Thunderbird, more on this below. I'm working from a Mac so some of this might not align exactly with what Windows users will encounter. For Windows users, the file path is something like this: [C:\Users\username\AppData\Roaming\Thunderbird\Profiles\] For macOS it's [Macintosh HD/Users/username/Library/Thunderbird/Profiles/***default.] You should find it easily from Thunderbird's main menu: Help>Troubleshooting Information. A new tab will open. Scroll down to the row "Profile Folder" click on "Show in Finder" (For Windows users it might be: Help > Troubleshooting Information - Under 'Application Basics' - about 8th in list - 'Profile folder' - click on 'Open Folder') Leave this Finder window open. Exit Thunderbird. In Finder, sort the profile folder by "Name". [View>Sort By>Name] this will keep the address books all together. Locate the files for the "Personal Address Book". You should see abook.mab.bak and abook.sqlite and any of your "custom" address books will appear as abook-2, abook-3, abook-4, etc. Some may appear as (imported address books) impab, impab-2, impab-3, etc.. Create a temporary folder on the desktop to receive copies of files. Open this new folder in Finder. This will make it easier to keep track of where you are at. Rename abook.mab.bak and abook.sqlite (It might warn you about changing format, but it's ok.) to something easy to remember. (I used abook-20.mab.bak and abook-20.sqlite because that number was higher than the # of abook files in my profile folder.) Now we'll fix each address book one at a time. Locate the first abook-* files. In our example we'll use 'abook-2. There should be 'abook-2.mab' and a corresponding 'abook-2.sqlite'. Move the 'abook-2.sqlite' to the temporary folder on the desktop. If the 'abook-2.mab' is 'abook-2.mab.bak', edit the file name to remove the .bak eg: so 'abook-2.mab.bak' is now 'abook-2.mab' and rename (edit the file name to remove the '-2') to 'abook.mab'. Restart Thunderbird. When you restart TB, the contacts in ''abook-2 will now be in 'Personal Address Book'. (Don't worry, you did not lose the original contents of 'Personal Address Book'. They are safe in abook-20.mab.bak and abook-20.sqlite. When we are all done, we simply remove the -20 from these two files and voila!, those contacts are back where they belong.) Exit Thunderbird. Look at the profile folder. You will see that restarting TB did three things: Converted 'abook.mab' to 'abook.mab.bak' Generated a new 'abook.sqlite file', Generated a new 'abook-2.sqlite' file, (but this abook-2.sqlite file is empty). Next, move the new 'abook-2.sqlite file' to trash. Rename 'abook.mab.bak' to abook-2.mab.bak'. Rename 'abook.sqlite' to abook-2.sqlite' Locate the next abook-* file. In our example we'll use 'abook-3. There should be 'abook-.mab' and a corresponding 'abook-3.sqlite'. Move the 'abook-3.sqlite' to the temporary folder on the desktop. Restart Thunderbird. Repeat these steps with each and every 'abook' and 'impab' file in the profile folder and all will be restored. Don't forget at the end to rename the original 'Personal Address Book' files, called 'abook-20.mab.bak' and 'abook-20.sqlite' in our example back to 'abook.mab.bak' and 'abook.sqlite' Remember to close (exit) TB while editing the profile folder. --