When the Edit menu > Preferences > Privacy > Firefox Will > Never Remember History menu item has been selected and Firefox has been restarted (the restart is immediately required once the menu item has been chosen), all tabs must be forgotten, not just hidden. Hiding means that changing the menu to Remember History, followed by a restart, restores the tabs in full view and and either creates or restores cookies associated with the tabs. That means Never Remember History is wrong. That's a privacy problem, especially because it is contrary to what Firefox is telling users when the Never option is current. An exception can be made for tabs that not only were open just before the menu change but which also were reopened when the menu change took effect, because that would notify the user of what is remembered, and the user vcan promptly close them, thus causing them to be forgotten. By not automatically reopening them, it would correctly be assumed that they've already been forgotten and won't return on their own, but, it turns out, they do. Test details are in bug 1201354, comment 10; and see subsequent commentary there. See also bug 513421.

Changing to Never Remember History does not erase history in FF 42.0 for OpenSuse 13.2. Restoring Remember History brings back tabs from before Never Remember History. If a tab is for a site that writes cookies, cookies appear. Since the tab is unsolicited, the cookies are unsolicited. I did the following experiment: From Use Custom Settings For History with Always Use Private Browsing Mode (with Show Cookies being empty and which disabled (dimmed) Keep (Cookies) Until and Clear History When Firefox Closes), I switched to Remember History (causing Use Custom Settings For History and with Show Cookies still empty), switched again to Remember History (doing it twice is different from doing it only once) (clicking Remove Individual Cookies showed only an empty cookie list) and visited amazon.com and waited for amazon.com to finish loading (tricky because it appears to be finished then loads more), which produced 7 cookies from amazon.com and 33 cookies from other domains (40 in total, one of which was from yahoo.com). I did not delete cookies or erase history. I switched to Never Remember History, causing Use Custom Settings For History with Always Use Private Browsing Mode (which disabled (dimmed) Keep (Cookies) Until and Clear History When Firefox Closes but this time listing lots of cookies, which I left), and switched again to Never Remember History. I clicked Clear All Current History and confirmed in the resulting dialog, which included Browsing & Download History and Cookies with a Time Range of Everything. I switched to Remember History. The Amazon tab reopened on its own. Lots of cookies were present, but they may have been newly recreated because the tab reopened, a clue to recreation being in the amazon.com session-token cookie, which is scheduled to expire at a time that suggests new creation, although 20 years minus 5 days in the future; repeating the process resulted in a slightly later time for the same date. Quitting and restarting FF resulted automatically in Use Custom Settings For History; my settings were not exceptional from the norm. I set Keep (Cookies) Until I Close Firefox and Clear History When Firefox Closes and only after that I turned on Always Use Private Browsing Mode. I have not figured out exactly what procedure actually forgets old tabs, permanently, without recovery, consistently. It seems that a procedure that works once does not work twice. We need a clear procedure that does this.