Many similar security problems occur if you leave computers turned on after logging in, and then walk away. Most Operating Systems or Desktops have options for setting screen locks or screen savers requiring a passoord as a method of reducing such risks.

Yes, as John99 says, if you require that people not be able to view saved passwords on your PC, then ensure you have a password and a really short screensaver (it activates after 1 minute of inactivity for example). But, even if this feature didn't exist, someone who was on your computer could still use your saved passwords by going to that website.

Of Course, but use password and view plain-text password is two different things.

Even if a master password is used then you can get access to the saved passwords once the MP has been entered during the current sessions and hasn't expired or you haven't logged off yourself from the Software Security Device.

• Edit > Preferences > Advanced > Encryption: Certificates > Security Devices: Software Security Device: Log Out button

On Linux it is usually easy to lock the screen via the Computer menu if you need to leave the computer unattended.