Due to privacy concerns, some Firefox users may wish to disable or turn off the session restore feature in Firefox. As an example, if your computer is used by multiple users, the browser could crash while you are checking your Gmail account. Rather than restoring Firefox, you decide to go do something else. When somebody else comes in to launch Firefox, the browser will be restored to your inbox (due to fact that is where it crashed). If you want a little more browsing privacy, here is how you disable the session restore feature in the browser.
First thing you will need to do is visit our old friend, the about:config screen. In the address bar, type in:
and then press the "Enter" key. If you are warned about the fact that you can really mess Firefox up if you muck around inside of about:config, just go ahead and click "I’ll be careful, I promise!" and continue.
Now, find browser.sessionstore.resume_from_crash in the list. You will want to double-click that entry, and then set it to false. Session restore inside of Firefox will now be disabled.
Here is what Mozilla has to say about the privacy issues that might exist:
Session Restore may keep you logged in to sites that you were logged into before you closed Firefox. If someone else used your computer after you, they could access your account on these sites. If this is a concern then you should not configure Firefox to open all windows and tabs from your previous session ("Show my windows and tabs from last time" option described above).
I would not consider this a huge privacy problem, but if it is something you are concerned about – it is nice to know Firefox left the option open for you.