The most useful feature with the worst UX. You have to type about:profiles and then create a new profile. But imagin you now want to move old profiles to a new computer and FF happens to run in a Flatpack. Yeah, much fun
I use firefox via flatpak and had no issues so far accessing profile data (in one of the folders in ~/.var/app/org.mozilla.firefox/.mozilla/firefox/ - I keep a regular archive of the entire folder as backup).
I don't see it yet, but hopefully soon.
It works really well though. Does exactly what I would expect and hope from such a feature.