In most cases, the add-on just blocks or hides cookie related pop-ups. When it's needed for the website to work properly, it will automatically accept the cookie policy for you (sometimes it will accept all and sometimes only necessary cookie categories, depending on what's easier to do). It doesn't delete cookies.
Not to keen on it accepting the cookies on my behalf, the point is to not accept the cookie. Consent-o-matic will actively deny the requests.
ublock was already mentioned couple of times. Additionally this one: https://addons.mozilla.org/en-US/firefox/addon/cookie-autodelete/
If you are going to auto delete cookies then I suggest consent-o-matic to auto deny cookie popups.
Oh right, there is this one is also currently installed, but hidden and never interacted with.
https://addons.mozilla.org/en-US/firefox/addon/istilldontcareaboutcookies/
Same functionality.
Not to keen on it accepting the cookies on my behalf, the point is to not accept the cookie. Consent-o-matic will actively deny the requests.