In this case, specifically, I am referring to the SiteSecurityServiceState.txt file in the profile root that seems to store security-related information for each website you visit upon Firefox exit.
Visiting a website in a private window does not cause it to be entered into this file, but visiting it in a private tab does, meaning that the file will contain a list of all sites that you browsed in private tabs.
In this case, specifically, I am referring to the SiteSecurityServiceState.txt file in the profile root that seems to store security-related information for each website you visit upon Firefox exit. Visiting a website in a private window does not cause it to be entered into this file, but visiting it in a private tab does, meaning that the file will contain a list of all sites that you browsed in private tabs.