See the user support documentation https://support.mozilla.org/en-US/kb/protect-your-thunderbird-passwords-primary-password#w_reset-your-primary-password-if-you-have-forgotten-it

Hi Matt, Followed the instructions in the link which you sent me. Was able to create a new password in Thunderbird. Clicked on SAVED PASSWORDS. The new password wasn't listed.

Arnold

Do you perhaps have an antivirus or other password vault type application which may be interfering in the process.

I just reread all of the discussion here, the link I provided allows you to clear a primary password. What the action does is actually delete all saved passwords. So, there will be no passwords in the password manager after you follow those instructions.

All account passwords that are stored in Thunderbird are set and maintained by the relevant mail providers, Thunderbird only remembers what you entered the first time you were asked and any time after when you are prompted for a password.