Please have also have a look at this:
Upgrading to Thunderbird 60.2.1 or later might cause your saved passwords to be ignored. The same problem can occur if you move the profile from one operating system to another. The workaround is to exit Thunderbird and delete the the pkcs11.txt file in the profile. Thunderbird will create a new version of that file when it restarts.
Found here:
http://kb.mozillazine.org/Password_not_remembered_-_Thunderbird