Contextual Identities on the Web
If we can get Tor Browser's first party origin feature in as well, this will be fantastic!  I would love to have the ability to type www.facebook.com and get a context that isn't linked to the rest of my tabs.I also want ephemeral containers so I could open a tab that forgets its cookies when I'm done.  Think private browsing but without forgetting my history, requiring a new window, or being limited to one context at a time.