DAMN Firefox's stupid "reopen last session" option ! I turned it off, it is now back on ! [View all]

This morning I had some kind of bizarre meltdown where Firefox kept trying to open new window after new window. Even when there were dozens of windows open, it kept opening more, slowing down as the processes ate up more and more memory. Response to the mouse and keyboard took several minutes. I had to click on the desktop and use Force Quit to get out. But if I try to relaunch Firefox now it starts doing the same thing ! I re-installed Firefox and the problem persists. Does anyone know where the file containing the last session info is kept ? Can I delete it without affecting bookmarks ? Any other hints ?

OOOOOPS ... I know better, but still didn't say I'm running this instance of Firefox under OSX 10.9.