Thunderbird can save passwords for each account, so you don't have to re-enter them when you check for new messages. You can view them and remove the ones you don't use anymore.

What if you share a computer, and don't want to prevent others from being able to see your stored passwords? With the Master Password feature, Thunderbird will require you to enter a master password in order to view or use the passwords you've told Thunderbird to remember. When you have a master password set, you'll only need to enter it once per session.

Defining a master password

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

1. At the top of the Thunderbird window, click the Tools menu (Alt + T) and select Options…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 Application menu button and choose Options…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 will appear. As you enter your master password, the Password quality meter will indicate the difficulty of guessing the password. Here are some tips for creating better passwords:
   • Use at least one capital letter
   • Use one or more digits
   • Use at least one non-alphanumeric character, such as: @ # $ % ^ & * ( ).
   • 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.

Now that your Master Passord is set, Thunderbird will prompt you to enter it the first time you need to use/view a password per session.

Changing or removing the master password

1. At the top of the Thunderbird window, click the Tools menu (Alt + T) and select Options…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 Application menu button and choose Options…OptionsPreferences.
2. Select the Security panel.
3. Select the Passwords tab.
4. Click Change Master Password.
5. Enter the current master 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. Click OK.

Forgotten Password

If you've forgotten your master password you can reset it.

1. Choose Tools -> Error Console.
2. Paste the following text:
   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)