Profiles have always been great, but it's kind of unfortunate that this feature seems to be locked behind a sign-in (the link in the article describes the UI as being in the profile menu).

I mean, I've been using about:profiles for ages, but it would definitely be nice to have a bit more polish (e.g. every now and again I forget that a newly created profile is automatically promoted to default)

[edit] well seems I have to eat my words - there's a switch in about:config named "browser.profiles.enabled" that toggles a profiles menu item with some UI that apparently has existed for years. Nice!