https://support.mozilla.org/bm/questions/1277793

I need the solution for mac os.

use --allow-downgrade on your command line

Which command line? Where?

I do not use apple products, but this is what google tells me about the topic. https://www.davidbaumgold.com/tutorials/command-line/

• first get the stable latest TB (Thunderbird) version from here: https://www.thunderbird.net/en-US/thunderbird/all/ • or, get older version from here: [https://support.mozilla.org/en-US/questions/1320204#answer-1380613 Download Thunderbird Installers]. • download DMG installer file. • run current TB > main menu > Help > Troubleshooting Information > about:profiles > "Root" directory . write down full PATH of root directory : it looks like something like this: "xxxxxxxx.profile-name" where x in xxxxxxxx is a random alphanumeric characters/numbers/letters, etc. this is your "''''current-TB-profile-name'''" folder, remember it. • close or exit from current TB. • goto below location: "~/Library/Thunderbird/Profiles/" which usually equals to: default HDD/SSD volume : "/Users/<YourMacOSUserName>/Library/Thunderbird/Profiles/" • write down the folder names which does not match "current-TB-profile". let say you found another TB-profile there : "'''old-TB-profile-name'''" folder. • in macOS "Finder", goto Mac default HDD/SSD volume : /Applications/ • select "Thunderbird" or "Thunderbird.app" , press Cmd+C buttons once. • goto "Desktop" in "Finder", create a folder called "Backup" , & go inside "Backup" folder. • press Opt+Cmd+V or Alt+Cmd+V to '''move''' current TB app inside "Backup" folder. • double click on DMG installer file, a new VOLUME will appear in "Finder". • '''copy''' the "Thunderbird" or "Thunderbird.app" from there , & paste into /Applications/ folder. • start macOS "Terminal" app (utility tool), and run below command to downgrade first: • start STABLE Thunderbird = TB in this way, with below terminal based command: '''"/Applications/Thunderbird.app/Contents/MacOS/thunderbird-bin" --allow-downgrade -p "/Users/<YourMacOsUserName>/Library/Thunderbird/Profiles/current-TB-profile-name"''' OR, start STABLE TB in this way: '''/usr/bin/open -n -a "/Applications/Thunderbird.app" --args --allow-downgrade -p "/Users/<YourMacOsUserName>/Library/Thunderbird/Profiles/current-TB-profile-name"''' • if all previous settings,emails,address-book,etc are back then you are using correct TB-profile , then skip below one bullet point. • if not using correct TB-profile , then exit from TB , & use below command to manually choose another TB-profile: '''"/Applications/Thunderbird.app/Contents/MacOS/thunderbird-bin" --allow-downgrade -p "/Users/<YourMacOsUserName>/Library/Thunderbird/Profiles/old-or-other-TB-profile-name"''' OR, start STABLE TB in this way: '''/usr/bin/open -n -a "/Applications/Thunderbird.app" --args --allow-downgrade -p "/Users/<YourMacOsUserName>/Library/Thunderbird/Profiles/old-or-other-TB-profile-name"''' • exit from TB. • remember which "xxxxxxxx.profile-name" is correct TB-profile, which you want to use. then, make it default TB-profile, so that using command-line again & again, is not necessary: • start STABLE TB this way: "/Applications/Thunderbird.app/Contents/MacOS/thunderbird-bin" -P OR, start STABLE TB in this way: /usr/bin/open -n -a "/Applications/Thunderbird.app" --args -P • a profile choosing window will appear, select CORRECT TB "profile-name" > also select "Use this profile as default profile" , etc similar option > ok • after setting default profile option , now you can start TB by clicking on the TB icon inside /Applications folder. More/Related Info: • [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 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 MAB/mork based Address-Book into other format]. • [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 goto the auto-update disable instruction page into another website.