This is the best cookie manager for developers that I've found so far for recent versions of Firefox. I previously used Cookies Manager+, before Firefox Quantum broke it. Cookie Quick Manager isn't a full replacement for Cookies Manager+ yet, but it's decent, and I sense that it will continue to improve.

One suggestion for the author: I wish that the "Delete all the cookies" action were harder to do accidentally. Perhaps a confirmation dialog or moving it to a menu would help. There are three trash can icons without labels and the "delete all" button has very wide reaching side effects.