If you can open TB, there is a method for resetting the master password:

http://kb.mozillazine.org/Master_password#Resetting_the_master_password

Otherwise, close TB, open the profile folder, located at

C:\Users\username\AppData\Roaming\Thunderbird\Profiles\8characterstring.default

(enable Hidden items on the View tab in File Explorer).

and delete or rename key4.db, cert9.db and logins.json.

You will have to re-enter account passwords if they were stored in the password manager.

Thanks for your help so far. However, when I entered the "chrome://ppki/content/resetpassword.xul" in the Error Console I didn't find an "Evaluate" button, with the updated version of Thunderbird.

Yes, I forgot that current versions of TB don't have an Evaluate button. But the method of deleting or renaming the 3 files should work:

https://support.mozilla.org/en-US/questions/1245204