I’m not really sure why extension preferences that are generated during installation are not deleted when the extensions are uninstalled. That’s in my opinion a serious flaw in the browser. Why would someone want to delete those old extension preferences? First to keep the preference file clean and tiny, second to limit privacy implications and third for better manageability.
ECleaner is not the first Firefox extension to delete old extension preferences in the Firefox browser. Firefox Prefs.js Cleaner and Firefox Preferences Cleaner have been two extensions with similar options.