Thunderbird, for your own security, does not remember primary passwords. If the primary password is lost, you can reclaim your accounts by these steps:

Go to tools>Developertools>error console 
- enter this text 

    openDialog("chrome://pippki/content/resetpassword.xhtml")  

- the Reset button should appear. All passwords for accounts will be deleted

Hi David, I have followed your instructions completely, but still cannot get in. I have also tried to re register as a new account, but still no joy. I have spent hours and hours on this, and just about ready to chuck the whole lot out the window. So cheesed off with the whole thing. Thank you for trying anyway. Corinne XXX

Did the reset button appear? That is sometimes difficult to see.

Yes I did see the reset button and followed your instructions, but still locked out. !!

That should have canceled all the account passwords. Is that what you're experiencing?

I still cannot get into my Thunderbird account, so do not know if I have any passwords left, or not. Being as it still will not let me in, I do hope that it has not wiped out all my passwords. ! Thank you for your efforts, but so far to no avail. X

Since restoring the account without primary password would also destroy all account passwords, My suggestion is to start afresh. If yours was a POP account, I would suggest, before deleting anything, to use Windows file explorer to copy the profile to another folder and then to delete the current Thunderbird AND also any remaining data. And then to install fresh. The message folders for the POP account could then be copied back to the Local Folders section so that they would not be lost. Let me know if this sounds workable for you. As far as account passwords go, you should be able to get new ones from your email provider. In summary, all is not lost. (TIP: Your old profile is in the folder at c:\users\<youid>\appdata\roaming\thunderbird\profiles. There may be more than one folder there, so you can get name of what was the active profile by viewing a small text file at c:\users\<yourid>\appdata\roaming\thunderbird\profiles.ini. If you need help in finding these files, let me know.