Firefox can save usernames and passwords that you use to access online services, such as banking and email websites. 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 once Firefox needs to access your stored passwords.

Defining a master password

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

1. At the top of the Firefox window, click on the Firefox button (Tools menu in Windows XP) and then click OptionsOn the menu bar, click on the Firefox menu and select Preferences...At the top of the Firefox window, click on the Edit menu and select PreferencesAt the top of the Firefox window, click on the Tools menu and select Options...On the menu bar, click on the Firefox menu and select Preferences...At the top of the Firefox window, click on the Edit menu and select Preferences...
2. Click the Security panel.
3. Check mark Use a master password.
4. 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. For more information on choosing passwords, see Choosing More Secure Passwords.
5. The password must be entered twice to confirm that you can enter it consistently.
6. To set the master password, click OK.
7. 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 Firefox window, click on the Firefox button (Tools menu in Windows XP) and then click OptionsOn the menu bar, click on the Firefox menu and select Preferences...At the top of the Firefox window, click on the Edit menu and select PreferencesAt the top of the Firefox window, click on the Tools menu and select Options...On the menu bar, click on the Firefox menu and select Preferences...At the top of the Firefox window, click on the Edit menu and select Preferences...
2. Click the Security panel.
3. Click Change Master Password.
4. Enter the current password to confirm you are authorized to change it.
   • If you've forgotten your master password, see Forgot my master password.
5. Enter the new master password twice.
   • To remove the master password, leave the new master password fields empty.
6. To accept the changed master password, click OK.
7. The master password is changed.

Based on information from Master password (mozillaZine KB)