THUNDERBIRD = '''TB'''. TB uses different TB-profile folders/sub-folders , those contains all emails,data,settings,etc,etc. all TB-profile sub-folders are inside/under this "''Profiles'''" folder, here: "~/Library/Thunderbird/'''Profiles/'''" that is usually located here: MacHDD volume: "/Users/<YourMacOsUserName>/Library/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" ''( where x in xxxxxxxx is random alphanumeric characters/letters/numbers, etc )'' ''( the "xxxxxxxx" is profile-folder-ID part , and "'''profile-name'''" is profile's name part )'' you '''MUST''' make a backup/'''copy''' of that .../Thunderbird/'''Profiles'''/ folder in your Desktop. then do whatever step you prefer or appropriate. ''( your old Mac computer , or your "Time-Machine"-software backup will also contain TB-profile folders )'' AV = Anti-Virus . FW = Firewall . SS = Security-Suite software. • some AV/SS creates insecure proxy/gateway for scanning/checking emails, but there are other secure way to scan/check emails. • user may have to [https://support.mozilla.org/en-US/questions/1315734#answer-1373915 Disable AV/FW/SS software], OR, enable+add+allow EXCEPTIONS in it (use your exact "mail-server" name/address). • if that does not solve, then see below: • in NEW MAC , you have to reinstall TB in below '''exact''' way: • use "Activity Monitor" tool/app in macOS, & completely STOP/KILL any running thunderbird. • download exact same version of TB , that you were using in OLD MAC . get DMG installer file. • ''get older version TB's DMG file from a linked page, that link is shown [https://support.mozilla.org/en-US/questions/1316147#answer-1372552 here] , then press Ctrl+F to find the link "Download Thunderbird Installers"'' • in '''Finder''' , copy (Ctrl+C) "/Applications/Thunderbird.app" , then goto Desktop , click on empty area once , press Alt+Cmd+V to '''move''' the app into Desktop, temporarily. • double-click on DMG file, new volume will appear in "Finder" > go into it > select Thunderbird or Thunderbird.app > press Ctrl+C. • go inside "Finder" , click on "Applications" once in left side > click on GEAR button in menu bar > select > '''Copy''' . • start "Terminal" tool/app in macOS: • use only one command from below TWO commands. • start TB with below command in this/below way once-only, to choose specific/earlier TB-profile: "/Applications/Thunderbird.app/Contents/MacOS/thunderbird-bin" -P • OR, start TB with below command: /usr/bin/open -n -a "/Applications/Thunderbird.app" --args -P • a small window will appear from TB, showing you list of available TB-profile(s) , choose earlier TB-profile or choose any profile , '''Do not select the option "Continue to use this as default profile"'''. TB will start with your chosen TB-profile, check & find out if this profile is uptodate or not , (does it have all older data/settings/emails,etc or not). if it is , remember the "xxxxxxxx.'''profile-name'''" portion, next time when you run TB again with same command , in profile window select the option to remember your choice & choose correct profile, as default folder. profile choosing window will show only '''profile-name''' portion , it will not show the alphanumeric "xxxxxxxx" portion. • exit/close TB. • start TB again with below any one command (out of two commands) in this/below way once-only, to choose specific/earlier & correct TB-profile: "/Applications/Thunderbird.app/Contents/MacOS/thunderbird-bin" -P • OR, start TB with below command: /usr/bin/open -n -a "/Applications/Thunderbird.app" --args -P • after profile choosing window appears, this time choose the correct '''profile-name''' and also select the option "Continue to '''use''' this '''as default profile'''", etc. • TB will start , wait for it do initial operations. then close it. • now you can click on '''Thunderbird''' or '''Thunderbird.app''' icon inside /Applications folder to start Thunderbird = TB. • you may/should disable auto-update in TB. ( when everything is back to normal as before, then you can update/upgrade, etc ) and by the way, all of your "'''Local Folders'''" are here: MacHDD volume: "/Users/<YourMacOsUserName>/Library/Thunderbird/Profiles/xxxxxxxx.<profile-name>/Mail/Local Folders/" you can copy-paste all contents inside these into another profile , but some sub-fodler's name will not or may not remain intact , you may+will have to test emails inside it & rename sub-folder if necessary . after transfer Local-Folders , you will need to restart the TB. EXTRA INFO for other/related problems/solutions: • [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. • [https://support.mozilla.org/en-US/questions/1316147#answer-1372552 Regain old data](in Windows, by using: downgrade process, or second-TB, or 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 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 based Address-Book into other format].