EDIT: Ignore my blind confidence. CAD is (mostly) broken in recent FF versions. (See ivn's reply to this post).
Consent-o-Matic with Cookie Auto Delete and Firefox's Multi-Account Container tabs covers it all nicely for me.
Cookie banners get handled, cookies I don't explicitly want to keep automatically disappear when I leave the site/close the tab, and those I do want to keep can be given their own containers to keep them separated.
You're right. I'll be damned. That'll teach me to set-and-forget then not keep up with changes to Firefox and their effects on extensions. Thanks for the heads up.
Set privacy.sanitize.sanitizeOnShutdown and privacy.clearOnShutdown_v2.cookiesAndStorage (I don't know if privacy.clearOnShutdown.cookies is still needed) to true.
To allow a website to keep cookies do CTRL+I on the address bar then check "Set cookie" in the Permissions tab.
I use uBlock Origin's picker mode instead. It lets you select which element you want block. It works on other annoying notices, popus and annoying stuff not just cookie notices