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 requir

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 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.