When I press "Let us know!" to report a site and in the confirmation dialogue check the box "Don't ask next time", the confirmation still comes up the next time.

It seems to be caused by line 34 in popup.js:

confirmationInput.addEventListener("input", () => {
    settings.skipSubmitConfirmation = confirmationInput.checked;
    GDPRConfig.setValuesFlags(settings);
});

GDPRConfig.setValuesFlags() does not exist, I think it should be GDPRConfig.setDebugValues().

I confirmed that making this change fixes the issue I was having on Firefox.