Protecting your internet passwords
Firefox can remember your internet passwords. This can be a very convenient option to use with all those different sites requiring passwords nowadays. However, if you use this function you have to set a master password, otherwise this feature is a real security threat. To enable a master password open your Firefox preferences and select the security icon. Check the "use a master password" box.
After launching Firefox is will ask you once for the master password, after that the internet password keyring will be unlocked. If the internet password keyring is unlocked, you can inspect all saved passwords in the Preferences -> Security -> "Saved Passwords ..." dialog. If you browse to a known website with a login form, the password is entered automatically.
Please note that at the time of this writing the implementation of Firefox' internet password keyring is not complete, as it is not locked automatically after a certain time of inactivity or before closing your laptop lid. If you want Firefox to lock your internet password keyring automatically after a certain time of you not using your computer, you might install the "Master Password Timeout" Plugin.