It's been awhile since I've used Mozilla and not Firefox, but if memory serves under the Tools menu there's a Cookies submenu. One of the choices on that submenu says something about managing cookies. Again if memmory serves, you'll get a window with two tabs on it, and one of the tabs will have a list of website addresses and their status as far as cookies (allow, allow session, deny). Not sure if you can change one from say deny to allow, but you should be able to delete sites that are set to deny that you want to allow, and then the next time you go to that site, it will ask you about cookies it's trying to set.
When in doubt, I find allow for session is a good choice. That means that Mozilla will delete the cookie when you close the browser. Great for sites that refuse to work if you don't accept their tracking cookies, etc. All you have to do is close the browser, and it's gone.
Hi. I cleaned out some of my uninvited cookies from my Mozilla browser and it has the choice of blocking future attempts of these sites to set cookies. I selected this setting but as usual outsmarted myself by not noticing that a couple of the cookies were from sites I use. I can't find how to reverse this command so if anyone has a clue on how to do this I sure would like to hear from them. Thanks for reading. Stewart