`about:profiles` is the UI. It allows you to create new profiles, restart Firefox with a specific profile, and manage profiles. Starting Firefox with `-p` (and no passed argument) starts a window which allows you to do the same.
Sorry I wasn't clear. Chrome profiles is a usable UI. Easy to find, switch, manage, create, delete, all with just a couple clicks, never very far away.
about:profiles is not discoverable (obviously), easily usable (you have to restart), or easily switchable (you have to open about:profiles or the terminal).
By the technical definition of "UI", the raw filesystem and terminal counts, sure. Opaque and unknown about: pages count, sure. But that doesn't make it good.
For features like this the UI is the feature. about:profiles is not a good UI, so the feature is 80% useless.