@yngve Sidekick is a Chromium based browser that also has the container-tab concept from Firefox, although now it is a paid feature of the pro version.
Profile-level separation is exceedingly cumbersome because users do not want to lose their bookmarks, sidebars, extensions, and other profile related data. We don't even want to go between multiple windows.
The common use cases I have seen are:
Developers needing to demo or test multiple personas at once
Consultants working for multiple clients where said clients have cloud platform providers in common (Office365, AWS, Github, ...)
Employees going through post-merger integration, where old and new companies are still in different tenants of cloud platform provider.
In all cases there is a need for the persistence that a private window does not provide, and the convenience of one profile and window.
Firefox is winning back some of this audience, and Sidekick is (presumably) profiting from them too.