@izacks It may claim to be rejecting 3rd party cookies but under the current schema it's not, unless you are accessing EVERYTHING currently on vivaldi.net and none on vivaldi.com (which I think may now be possible.) It is not, or should not be, possible to log in on two linked domains unless cookies from both are accepted. It is a security vulnerability to do so. It is just possible, I suppose, that firefox is treating them as un-connected domains, which it should not - as they cross-reference each other.