Thunderbird = TB . v68 series TB uses mork/mab based files for Address Book . v68 cannot use sqlite based Adrs-Book. v78 series TB uses sqlite based files for Address Book . v78 cannot use mork/mab based Adrs-Book, (though it can convert mab/mork into sqlite on 1st time TB-profile upgrade/update process). so if your earlier TB was a v68 series TB , then please use a v68.12.1 to recover/regain all old data/settings 1st , later you can update/upgrade. if your earlier TB was a v68 series TB , and you tried to use v78 series as newer TB & old profile directly , then that would create problem : you need to start TB by using the -P profile choosing option, change "*''.''mab''.''bak" files into "*''.''mab" files, etc, etc, etc. if your earlier TB was a v78 series TB , and you tried to use v68 series as newer TB & old profile directly , then that would also create problem : you have to use "--allow-downgarde" option to start TB 1st , then use that with -P option & choose correct old profile , to properly downgrade a TB-profile. PLEASE MENTION , WHICH WAS WHICH VERSION OF TB , IF POSSIBLE. what version you were using earlier. what version is newer version. etc . plese provide related more info, so supporters & volunteers can understand the problem, the less info you will give, more harder it will be , to solve it. ''(do not post "personal" or "private" info)'' More/Related Info: • [https://support.mozilla.org/en-US/questions/1315734#answer-1373915 Disable AV/FW/SS software] mail-protection OR enable+add+allow EXCEPTIONS in it & add mail-server names/address . Some AV/SS creates insecure proxy/gateway for scanning/checking emails, but there are other secure way to scan/check emails . Please make sure the option "Allow AV to Quarantine email" is selected inside TB, and make sure your AV/SS is set to Scan all files on access. • [https://support.mozilla.org/en-US/questions/1316147#answer-1372552 Regain old data in Windows] : by using downgrade process, or by using a Second-TB, or by using manual file-transfer,etc). • [https://support.mozilla.org/en-US/questions/1304550#answer-1352743 Downgrade TB in macOS] : [https://support.mozilla.org/en-US/questions/1317224#answer-1374579 downgrade by using second-TB] , [https://support.mozilla.org/en-US/questions/1318142#answer-1377801 upgrade/downgrade TB]. • [https://support.mozilla.org/en-US/questions/1315734#answer-1373022 AT&T , ATT-Yahoo related settings for TB]. • [https://support.mozilla.org/en-US/questions/1304518 Yahoo , ATT-Yahoo related settings for TB]. • [https://support.mozilla.org/en-US/questions/1316618#answer-1374784 Microsoft Hotmail/Outlook related settings for TB]. • [https://support.mozilla.org/en-US/questions/1317619#answer-1375337 Convert MAB/mork or SQLite based Address-Book into other format], mbox email files into other format. • [https://support.mozilla.org/en-US/questions/1316147#answer-1372552 Disabled auto-update or update of TB] : inside the linked page , click on the topside 1st link to goto the auto-update disable instruction page into another website.

A Personal Address Book that doesn't accept changes or additions is usually corrupted, and one way to recover is to export the book to an LDIF file (Tools/Export in Address Book), open the profile folder (Help/Troubleshooting, Profile Folder, Open Folder), close TB, delete or rename abook.sqlite (TB 78) or abook.mab, restart TB, which will automatically create an empty, new Personal Address Book, import the LDIF file from Tools/Import, then move the imported contacts back to PAB by drag and drop. A similar process works for Collected Addresses (history.sqlite or history.mab).