Firefox saves your preferences and settings in a single place. Some of those settings you can alter in the Options dialog box, but not all of them. The others require going into an obtuse, difficult-to-understand work environment called about:config.
And if you play around in about:config, changing settings that you think you understand, you can really mess things up. That's why, when you enter this environment--which you do by clicking the address bar (pressing CTRL-L will not work in this situation), typing about:config, and pressing ENTER--you have to click a button that reads "I'll be careful, I promise!"