If it is really the Thunderbird master password it is going to ask for the password for the software security device. Is that what you are seeing? Here is the info on the Thunderbird Master Password. It describes how to clear it. https://support.mozilla.org/en-US/kb/protect-your-thunderbird-passwords-master-password Take note that is is going to clear all passwords including and passwords to your email accounts so be sure you know what they are or how to reset them with your provider.

maselfe,

How did you solve your issue?

And did you determine how master password got set? (It doesn't set itself)