It should be normal behavior for browser to delete expired cookies at idle time, and chrome does this but firefox does not. You need to manually delete all cookies after some time to make sure expired ones are gone and relogin and whatever else thats stored in cookies. Problem with this is site data is treated as cookies and some websites by design generate a lot of data in profile/storage/website+name folders and absence of cleanup algorythm causes folder to grow without control.
currently i have 6GB of site data and there is not even an option to clean it on exit like cache because cookies and site data are same checkbox.