As you know all your usernames and passwords are stored in your browser which is insecure when someone uses your computer they can see them easily. To protect this Firefox provides an option. Go to Tools -> Options -> Security tab -> check the box that says “Use a master password”. This will open up another box asking you for your master password – type it, and make sure you remember it.
To see how it works go to Tools -> Options -> Security tab and then under the Passwords section hit Show Passwords button it should ask you for your master password. Only when you enter the master password it will show you your passwords.