Which URLs keep Google's cookies?

[If this is off-topic for this community, mods please let me know and I'll delete it.]

Edit: Deleting all cookies by hand seems to have solved the problem. Probably related to the bug pointed out by @trackd@lemm.ee in the comments. Cheers!

My browser has an extension that deletes cookies left by any website as soon as all tabs with that website's domain are closed. I can whitelist some of course, but Google's domains (*.google.com,*.gmail.com) are not whitelisted.

What's strange is that if I sign into Gmail, then close the tab or even Firefox, when I go into Gmail again I'm signed in automatically. I have no automatic sign-in functionality in Firefox, so this must happen because Google is saving cookies somewhere – or am I wrong?

So I don't understand with which URL these cookies are associated with – it can't be *.google.com, because otherwise they would have been deleted.

Can anyone enlighten me about this?

[Not sure I've been able to explain myself clearly; apologies and let me know in case.]