Thunderbird can save usernames and passwords that you use to access your mail accounts. If you share a computer with anyone, it is recommended that you use a master password.

After you have defined a master password, you will be prompted to enter it whenever Thunderbird needs to access your stored passwords.

Defining a master password

By default, Thunderbird does not use master passwords to protect stored credentials. To define a master password:

1. At the top of the Thunderbird window, click the Tools menu and select OptionsIn the menu bar, click the Thunderbird menu and select PreferencesAt the top of the Thunderbird window, click the Edit menu and select Preferences, or click the menu button and choose OptionsPreferences.
2. Select the Security panel.
3. Select the Passwords tab.
4. Put a check mark next to Use a master password.
   
   
   
   
   
5. The Change Master Password dialog appears. As you enter your master password, the Password quality meter indicates the difficulty of guessing the password. To create a quality password, your master password should include the following:
   • At least one capital letter
   • One or more digits
   • At least one non-alphanumeric character, such as the following: @ # $ % ^ & * ( ).
   • Also, remember to make your password something that is easy for you to remember, but difficult for others to guess to ensure that you, and only you, will be able to retrieve this secure information
6. The password must be entered twice to confirm that you can enter it consistently.
   
   
   
   
   
7. To set the master password, click OK.
8. The master password is defined.

Changing or removing the master password

You can change or remove your master password at any time:

1. At the top of the Thunderbird window, click the Tools menu and select OptionsIn the menu bar, click the Thunderbird menu and select PreferencesAt the top of the Thunderbird window, click the Edit menu and select Preferences, or click the menu button and choose OptionsPreferences.
2. Select the Security panel.
3. Select the Passwords tab.
4. Click Change Master Password.
5. Enter the current password to confirm you are authorized to change it.
6. Enter the new master password twice.
   • To remove the master password, leave the new master password fields empty.
7. To accept the changed master password, click OK.
8. The master password is changed.

Forgotten Password

If you have lost or forgotten your master password or you want to disable the feature, you can reset the master password. Resetting the master password will remove all stored password information. Upon resetting, you will lose all the stored information in the Password Manager, as this is a built-in security feature to prevent people from simply resetting your Master Password to gain access to your passwords.

1. Choose Tools -> Error Console.
2. Paste the expression
   openDialog("chrome://pippki/content/resetpassword.xul")
3. Press the Evaluate button.
4. Accept when prompted with the confirmation dialog box.

Based on information from Master password (mozillaZine KB)