OK I still don't know what to have a checkmark in if I want it to not clear my Password but I do want it to clear Histry.
In Firefox's menu bar, go to "Tools" and click "Options...".
In the Options window, go to the "Privacy" tab.
In the "Private Data" section at the bottom of the window, click "Settings...".
In the Clear Private Data window, uncheck the box next to "Saved Passwords".
That should keep Firefox from clearing your passwords.