After shutting down Firefox and restarting, I tried the new profile manager UI just now and also thought I had lost my existing profile. Luckily, it was still available from the hamburger menu (just below the “Synchronise all your devices” option) and I was able to switch back to it (with all my existing bookmarks, containers and extensions).

I'm glad you found yours, but mine was gone. I went spelunking through the profiles directory and it was nowhere to be found.

I had to pull it from backups.