If you have ever got up and left a Private Browsing session unattended, your private information is sitting there for any passer-by to see. With PB Exit, a Private Browsing window will close after 10 minutes of inactivity, either the last page load or tab switch.
This Firefox extension could not be easier to use; simply install it and let it run in the background. In my trial, it worked as advertised, closing Private Browsing tabs after I let them sit idle for 10 minutes. There are no settings to muss with, though the developer has a note on the extension's description that he may "add settings in the future (e.g., customer idle time)."
If you are a regular user of Firefox's Private Browsing feature, PB Exit is a handy fail-safe to keep your data away from prying eyes.