I'm uncertain if it's a bug or if it's platform/build specific. If you are deeply paranoid you may perform a TrueCrypt encryption over your Firefox profile.)

A smart geek can discover what you have browsed even after you clear all history data by using a data recovery program on your hard drive, so disabling session restore itself does not effectively protect your privacy at all. (A separate 'guest' Windows user would not be able to access other users' Firefox profiles, I believe. This is the safest and permanent solution, and additional keeps your history data while preventing inexperienced users from looking at them. Or keep a separate profile or even separate Windows user account, and just offer others a guest Firefox profile or user account. If you consider what you are browsing a secret, please use Private Browsing Mode.