Cross-site cookies are accepted without being asked. How can this be prevented?

Hello everyone, I keep having the issue that on some websites, a whole list of cross-site cookies is displayed when i click the icon to the left of the address bar. "These parties can use cross-site cookies and site data while you are on this site." Especially Google (I am not logged into a Google account) lists many different sites here. I can then click on "Allowed X" - but after a few days, the entry appears again without me being asked! Under "Privacy & Security" I am using the Standard level. It says there: "Firefox blocks the following: (...) Cross-site cookies in all windows (...)" So why are some still being allowed? How can I generally prohibit them except for those I manually allow? The five levels for cookie handling under "Custom" are unfortunately not very self-explanatory, and I can’t find any whitelist option there either. Thanks for any advice! Best regards, Martin