Moreover, using Cookie Monster you can allow cookies for second level domain names. Meaning that if you navigate to mozilla.org, cookies for www.mozilla.org will be accepted — but cookies for adnetwork.com will not.
This feature makes many sites work which otherwise break with third party cookies disabled, while still discarding the majority of third party cookies.
I don't think that that option accomplishes the same, or is even similar.
What about sites that you never visit (= type in address bar / follow links to) directly, but which are on some subdomain of the visited site nonetheless? Cookiemonster will accept the cookie. Firefox, with your config, will not.
What about sites that you do visit (eg facebook.com) but don't want to accept/send third party cookies for? Firefox, with your config, will. Cookiemonster won't.
This feature makes many sites work which otherwise break with third party cookies disabled, while still discarding the majority of third party cookies.