Firefox should have an option to open multiple instances having separate profiles. Instances should not share any user data.
Edit: The ability to open more than one Firefox profiles simultaneously
Firefox should be able to open multiple profiles simultaneously. All profiles should have their own seperate containers. The cookie created in one profile cannot be accessed by any other profile. No user data should be shared among the profiles, containers.
One solution to do this is to use nested tabs. That is the first layer of tabs should be for profiles. And each profile than will be having normal web page tabs with in them.