Not sure if this is what you are looking for but I use the Cookie AutoDelete extension and you can whitelist domains so they are not deleted.
Firefox
/c/firefox
A place to discuss the news and latest developments on the open-source browser Firefox.
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3. Communicate in a civil manner
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See "Manage Exceptions" (Privacy in Settings).
Typically, when accessing an "exception web page" you'd (from address bar/^i) open permissions and specifically ALLOW cookies for that site.
I use the cookiebro extension for that. Allows whitelisting domains or single cookies and can clean up all others with a few clicks.
Since firefox' total cookie protection there's not much mileage in deleting cookies at all really.