I think this is not nessecary on Desktop, the "Data protection-Cookies and Website data - Manage exceptions" should deal with that.

Essential on mobile, to delete all cookies apart from selected sites. I integrated it to my custom addon collection

https://addons.mozilla.org/de/firefox/collections/17446767/Fenix-Addons/

it works pretty great! The UI is usable although a little small, and Addons are able to modify cookies and site data on Firefox Mobile.

A mobile interface with bigger buttons would be great.

I will ask the developers to add the addon among others to the official list.