I believe this is part of the "Prevent cross-site tracking" feature. I do know that Webkit/Safari has had this feature for a while now, under the name "Partitio

I believe this is part of the "Prevent cross-site tracking" feature. I do know that Webkit/Safari has had this feature for a while now, under the name "Partitioned storage." Safari has a handful of other policies under the "Intelligent Tracking Prevention" banner, like blocked or ephemeral cookies for non-first-party domains. Firefox is playing catch-up with this feature. The announcement says "...making Firefox the most private and secure major browser available across Windows and Mac." Note the part that I've emphasized.