In the Firefox tab Privacy in Preferences we can set Accept cookies from sites and then Keep until: they expire, I close Firefox or ask me everytime.
Currently I have keep cookies until they expire but I'd like to be able to select a time frame, for instance 40 days; so cookies with be automatically deleted once they lived for 40 days. Is this possible? I couldn't find an option for this in about:config but perhaps I didn't look in the right place.
"network.cookie.lifetimePolicyto" and "network.cookie.lifetime.days are what you are looking for. Kudos to Echo419@superuser.