I'll add a screenshot now, otherwise it might not be clear

This is displayed when I open a private browsing session although it should be set to zero:

Cross-site cookies are automatically allowed in some circumstances. You can prevent this by changing Tracking Protection to Custom with Cookies set to block "All cross-site cookies (may cause websites to break)".

Make sure you do not sign into your Google account in the private window.

That's not the matter: I close the browser -> reopen it -> open a new private session -> go to google.com and its still there!

These cross-site cookie permissions are automatic and does not indicate cookies are being retained across sessions.

Make sure you close all private windows or quit browser with Command+Q to end the private session.

Yes, that's right, no cookies are saved, but these cross-page cookie permissions are saved and the idea behind the private mode is that you always start from scratch and not with the last state.

The permissions are generated automatically from scratch, you can disable cross-site cookies if you are concerned.

While cross-site cookies from trackers are blocked in Firefox by default, a site may signal to the browser that it needs to use them for important functionality. In this case, Firefox will allow a third-party website to use cross-site cookies the first five times (or up to 1% of the number of unique sites you visit in a session, whichever is larger) without prompting you.

Hi, I know that from your manual, that's not the point. I'm developing a website and if I try to test a certain behaviour, I can only work if I can reproduce a initial state (empty default browser).

And those permissions are not erased after creating a new session!

You can install Developer Edition for testing an empty default profile.

Hi, thanks looks nice, I'll try it when I have time. On the other hand, I use incognito mode on Chrome and it works fine there...

I'm interested in where I can reset these cross-page cookie permissions in Firefox?

Do the cookie permissions not appear on first run in a new profile or Developer Edition?

Do you see any difference if you clear all history (Ctrl+Shift+Del > When > Everything)?

Can you provide a link to a public site with steps to reproduce the issue?

I've sometimes reproduced it with istockphoto.com, but only when I've opened istockphotos via a Google result and it seems to be necessary to use the session for a while until it set's that permission. (I don't always want to compare you, but it works in Chrome ;)