A master password can keep you secure by requiring it to be entered in order for Firefox to access your stored passwords that particularly comes in handy if you share your computer with anyone. To create a master password, click on the Menu button, then go to 'Options->Security tab'. Check the box next to 'Use a master password' then follow the directions in the pop-up window. You can also disable or change your master password through this same window.
If you find yourself in the unfortunate position of having lost your master pass, you can re-set it in a roundabout way. This action will remove all your saved usernames and passwords though. To reset it, enter 'chrome://pippki/content/resetpassword.xul' into the location bar and press Enter, then click on 'Reset' on the ensuing pop-up page. You can then go and create a new master password from there using the above directions.