You are right; I should have fully read OP's post before advising.
In my case, all cookies (except the ones I marked as exception) are deleted when browser is closed. Note, 3rd party cookies are by default blocked on Firefox.
Meta is actually trying to comply circumvent EU's strict privacy policy.
Recently, The EU's top court said in July that Meta must first get consent before showing ads to users - a ruling that jeopardises the company's ability to make money by tailoring advertisements for individual users based on their online interests and digital activity.
I personally consider this[1] to be the ultimate test of Browser fingerprint protection coverage.
Let me know if you manage to find a way to defeat this test.
In my opinion, Firefox should give an option to enable ECH forcefully for users like me who has AdGuardHome/Pi-Hole running on Home Network.
Currently, if DOH is disabled in Firefox setting, ECH won't work, as per Firefox. 😦
That was my assumption as well. Thanks OP for sharing.