[Liquid_Fire]

I think you may be mixing up profiles and containers.

Profiles do have a built-in UI at about:profiles or by launching Firefox with -P, neither of which requires an extension. Admittedly this UI is a bit basic, but a better version is being rolled out (https://support.mozilla.org/en-US/kb/profile-management). Running multiple profiles side by side does indeed involve running multiple instances of the browser.

Containers are an internal API and need an extension like Multi-account Containers to provide a GUI (though this is an official extension by Mozilla), however they don't require running multiple copies of the browser.

[ReadCarlBarks]

The new profile manager has already been enabled for most users.

If you don't see it in the main menu yet, type `about:config` into the address bar and toggle `browser.profiles.enabled`.