perhaps https://thunderbirdtweaks.blogspot.com/2019/09/i-lost-my-profilemail-on-update-to.html this is still happening with 78

'''TB''' = Thunderbird. • user may also need to [https://support.mozilla.org/en-US/questions/1315734#answer-1373915 Disable AV/FW/SS software], OR enable+add+allow EXCEPTIONS in it . some AV/SS creates insecure proxy/gateway for scanning/checking emails, but there are other secure way to scan/check emails. • if that does not solve, then see below. • please, you MUST first '''make backup/copy''' of ALL folders+files under this location, copy into '''Desktop''': "%APPDATA%\Thunderbird\Profiles\" that is usually located here: "C:\Users\<YourWindowsUserName>\AppData\Roaming\Thunderbird\Profiles\" that ...\Thunderbird\'''Profiles\''' folder contains multiple sub-folders , those sub-folders are various TB-profile folders , a TB profile folder looks like this : "xxxxxxxx.profile-name" • as you want to recover back all previous items, so i suggest to 1st downgrade back to v68 series TB . follow downgrade steps shown inside download-link in below. • you may continue to use v68, just disable AUTO-UPDATE option in v68 series TB. instruction for disabling auto-update is shown in downgrade page's top side in 1st link. • later, if you want to upgrade the v68 series TB into v78 series TB, then follow the upgrade link shown in below. Some useful links: • [https://support.mozilla.org/en-US/questions/1316147#answer-1372552 Disabled auto-update of TB] : inside the linked page , click on the topside 1st link, to go into the auto-update-disable instruction page, into another website. • [https://support.mozilla.org/en-US/questions/1316147#answer-1372552 Regain old data by downgrading TB] in Windows , [https://support.mozilla.org/en-US/questions/1316147#answer-1372645 downgrade by using a second-TB] , or downgrade by using manual file-transfer, etc . [https://support.mozilla.org/en-US/questions/1318142#answer-1377801 upgrade-TB]. • [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 TB]. • [https://support.mozilla.org/en-US/questions/1315734#answer-1373022 ATT-Yahoo related settings for TB]. • [https://support.mozilla.org/en-US/questions/1304518 Yahoo related settings for TB]. • [https://support.mozilla.org/en-US/questions/1316618#answer-1374784 Microsoft related settings for TB]. • [https://support.mozilla.org/en-US/questions/1317619#answer-1375337 Convert mork/mab based Address-Book into other format].