You can view using a master password as a way to authenticate who you are to the Software Security Device, just as you do with a server on a web site: you log into a web site and enter your credentials and you do the same if supplying the master password.

Using a master password is not selected by default you will need to set one in the Password Manager, as explained below under Setting a master password. Its usually trouble free, but if you run into a problem with the master password frequently the only workaround is to delete it, which will delete the stored passwords. If you decide to set a master password write down a copy of your passwords somewhere safe beforehand. Alternatives to the built-in Password Manager such as Keepass or Lastpass provide their own implementation of a master password. However, a master password will not prevent anybody else from reading locally stored e-mails, reading your browsing history, or from accessing sites the browser is already logged in to. Its also a good idea if you installed S/MIME certificates. If you do that the passwords will be stored encrypted, and anyone using your profile will be prompted to enter the master password when access to the stored passwords is needed. Its recommended that you set a master password if anybody else has physical access to your PC. If you step away from your PC for a moment it only takes about 15 seconds for somebody else to see your passwords.

The passwords can be easily viewed using Firefox or Thunderbird menu commands. However, it stores the passwords unencrypted in a database file in the profile. The Password Manager can be used to automatically fill in the username/password needed to access web sites and log into mail servers. (Redirected from Thunderbird : FAQs : Reset a Master Password)

Thunderbird master password reset