if same TB once showed you all what you needed , and then in second time not showing this , then it looks like to me that : you need to choose the correct TB-profile again in TB. TB = Thunderbird. usually ALL TB-profile(s) are here: "%APPDATA%\Thunderbird\Profiles\" that is usually located here: "C:\Users\<WindowsUserName>\AppData\Roaming\Thunderbird\Profiles\" inside that "Profiles" folder , there are very likely multiple TB-profile sub-folders , they look like this : "xxxxxxxx.profile-name" , where x in xxxxxxxx is a random alphanumeric character/number/letter, etc. to migrate , you suppose to copy above location's all files+folders into same location of another computer. you MUST use exact same version of TB , in source & destination computers. later, in destination, you can change/update/upgrade TB version. (keep the backup/COPY of all TB-profiles) then (in destination computer), use a command-line like below , to start TB , to choose specific TB-profile: start windows "Command-Prompt": "C:\Program Files\Mozilla Thunderbird\thunderbird.exe" -P above command will show a Profile choosing window, select a profile , check if that is your earlier profile which you want-back or not, if not then exit TB, again start TB with above command, & select another profile. when you will find the correct profile that you want to use & have all previous emails+settings, etc , then remember the profile name which you selected in profile-choosing window. exit TB. start TB again using above shown commandline. this time select the correct profile , and also select this option "Use this profile as '''default profile'''",etc similar. once you do above step , then you don't have to start TB with a command-line anymore. you can use TB's icon/shortcut to start TB. some AV/SS software conflicts with thunderbird's connection with external mail-servers , so see below link, related to AV/FW/SS. do not do auto-update during these migration . disable auto-update , see below link on how to disable auto-update. if you ahve the backup copy of all data from source computer , then again install a fresh TB , exact SAME VERSION in new / destination computer . again use the TB-Profile , this time OVER-WRITE into destionation computer . and again start with shown commandline to choose correct profile. 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 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.

Dear atErik, Thank you for your complete and helpful reaction! You mentioned one thing that was not in the directions in "Moving Thunderbird Data to a New Computer": The version of Thunderbird on both computers must be the same! I came to the same idea after two hour of unsuccesful trying, whereafter I posed my question and went to bed. This morning before anything else I tried again; I had an old version of TB somewhere in my (large!) directory "software", and everything went fine. So first of all: thank you! Thank you very much! Your answer contained a lot of useful information, so I stored it in my software/thunderbird folder.

thank you. credit is due to the spirit of "open-source" , & to community (devs+supporters+users+people) who are keeping borrowed knowledge distributed & improving further & sharing it.