Thunderbird 78 will change the address books. All *.mab files will get formatted as *.sqlite files. All the *.mab files will then get extension *.mab.bak. There have been a few cases where people have experienced issues with address books and needed to perform a few jumps through hoops to get all working again. Also, when a profile gets updated to be used by a new version it's not so easy to go back again, although it can be done.

I'm a bit surprised you did not get updates a long time ago to version 68.12.1 which was the last in the 68* series.

I would recommend that you create some backups. Export each address book as LDIF or *.csv file - as a backup - just in case. Exit Thunderbird and copy the .thunderbird folder that contains all the profiles to an external hardrive. Put it in a folder called eg: TBv68100 Linux and Unix Profile folders are located here:

   ~/.thunderbird/<Profile name>/ 

However, if you're using a third party build from Debian or Ubuntu, those builds store your profile folder here:

   ~/.mozilla-thunderbird/<Profile name>. 

Once you have a backup- uninstall thunderbird.

Then get a fresh up to date download from link choosing prefered language and correct OS. https://www.thunderbird.net/en-US/thunderbird/all/

Please note: If 'Tasks' are an important part of your use of Thunderbird, there is a glitch at the moment. If you have an old but repeating task, where many past tasks were completed, it is losing the 'completed' status. Meaning that old task is showing all the repeats as not completed. Bug report is being processed.

Hi Toad-Hall Thanks very much for taking the time to provide a detailed answer!

I followed your instructions carefully, and it worked perfectly.

At the end of your instructions i just deleted the contents of the new profile folder, then copied the contents old profile folder into it. All accounts and email folders are there and working.

Very well done :)

James

Hi Toad-Hall Thanks very much for taking the time to provide a detailed answer! I followed your instructions carefully, and it worked perfectly. At the end of your instructions i just deleted the contents of the new profile folder, then copied the contents old profile folder into it. All accounts and email folders are there and working. Very well done :) James

Do you have Firefox 78 installed on Mint 20.1, or are you just running it out of the extracted folder? If you got it to install, how did you do it? I read what Toad-Hall wrote. I went to the page and downloaded the zipped file. I can run it directly out of the folder, but I would like to have it installed. Thanks

I'm guessing you meant Thunderbird 78 when you wrote Firefox 78?

I'm just running it from a sub-folder in the home folder. I created shortcuts on the desktop and menu and bar. I saw no reason to spend more time trying to install it, as it runs just fine from there.