Wanted this for years, and was one of the blockers that made Firefox impossible to really use. It looks like they're copying Chromium's UI (which is good, since it was superior to Firefox's about:profiles, for reasons others have mentioned).

But neither this post, nor the splash screen which links to this, nor any of the menu options, actually say how to use "profiles". I can't find it in an internet search either, instead coming up with Firefox's old about:profiles solution. Nor is it enabled using the little user icon in the browser bar, nor is it activated with the same shortcut as Chromium profiles.

I eventually found it in their knowledge base, and it appears to be locked behind a sign in button under the profile icon? Or my (up to date) browser lacks the button? This is such a confusing experience.

Here it is for anyone else who struggled:

https://support.mozilla.org/en-US/kb/profile-management

Some folks under https://news.ycombinator.com/item?id=45829275 seem to say that there's an about:config to force enable it, or you could wait for it to get rolled out to you.