This is a little hard to explain, but when you start up Firefox normally, your regular (non-private) windows all share one cookie jar. When you open a private window, it starts a second cookie jar for the private windows. If you keep that private window open and open a second one, you'll see that sites you logged into in the first private window are also logged in in the second private window. Only when you close all the private windows does the second cookie jar get destroyed. You can set Firefox to start up in a private window (no regular windows), but you will not be able to use the same trick of having two cookie jars because all the private windows will share one cookie jar. You would have to completely quit out of Firefox to destroy that cookie jar. Does that make sense? This article has a section on how to set up automatic private browsing on the Preferences page, Privacy panel: [[Private Browsing - Use Firefox without saving history]] -- the "Can I set Firefox to always use Private Browsing?" section.