Another add-on besides 'Cookie Culler' is "Selective Cookie Delete": https://addons.mozilla.org/en-

Another add-on besides 'Cookie Culler' is "Selective Cookie Delete": https://addons.mozilla.org/en-US/firefox/addon/selectivecookiedelete/

Both are very good but in different ways. I use Selective because it seems to be less work. And it hasn't failed me yet. Just be sure to go into Options / Privacy / Settings and UNclick clearing cookies.