Firefox has this great new feature, "Session Restore", that remembers where you were, and if your Firefox crashes, restores the tabs to the state they were before you start.
I didn't like it.
To turn it off:
1) Type "about:config" in the URL Navigation bar
2) Right click on the window and select "New->Boolean"
3) Type "browser.sessionstore.enabled" and set it to "false"
This is from the mozilla wiki.