b. Now you're being presented with a warning. Ignore it and click on the button "I accept the risk!".
c. Type in the filter bar: sessionstore
c. Double-click on the item called browser.sessionstore.interval. The default interval is 15000, which means 15 seconds. Add three zeros to the existing value, so that it becomes: 15000000 and click the OK button.
d. Close Firefox and launch it again. Now you've practically disabled the session restore feature.
Note (1): Leave the other cache and sessionstore settings as they are: usually, the less invasive a hack is, the better. Because this reduces the risk of unexpected unwanted side effects.
Note (2): This is a user preference. Repeat this hack in each user account.