I have had very few notices about Thunderbird upgrades. My records show Setup numbers: 3.0.exe, 8.0.exe, 52.4.0.exe, 52.9.1.exe, 78.12.0.exe, and then 115.5.2.exe. My address book made it through all but the last upgrade. No notice that I should save a copy of my address book ever showed up in any correspondence. I tried reinstalling an old upgrade but the upgrade always wants to update the installation. I did try to reinstall 52.9.1, but the next screen wanted my name and password. It would not accept any name and password that I could remember. So I tried searching for them at Xfinity and Comcast to no result. I did not try my Firefox password because it didn't seem appropriate. Incidentally, my records show 135 upgrades for Firefox for the same length of time as the upgrades for Thunderbird. My fear is that my address book is unrecoverable. I chose Thunderbird because I ofigured none of it would need backup if it became necessary to reinstall Windows 10. Which happened last year. I did not expect Mozilla to change Thunderbird so drastically that I couldn't get every thing back. Sorry but I'm a senior citizen and not computer literate. If I have to try to recreate my address book I may as well use Microsoft's Outlook as it looks somewhat simpler than Thunderbird - which I doubt.