Basic Firefox cookies settings

Due to a law changing in my country, many sites have completely disabled the "Reject all cookies" button. Now they are offering the option to allow all cookies and "Reject all cookies if you pay a monthly subscription/create account". This is awful, I'm fed up and I'll begin using scorched earth tactics. I use Mozilla Firefox with some add-ons already. What would happen if I change Firefox cookie blocking settings to "Block All third-party cookies (may cause websites to break)"? …

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