Seems quite similar to Zen's experience, except it seems to be missing folders (which I admittedly don't use often, but they're sometimes handy to group a Jira ticket with a PR, or similar). I'll probably still stick with Zen while it's around, and maybe I'll hop over to LibreWolf as I'm not too happy about Mozilla's recent stance on privacy.

Is it similar to tab groups? It's available on Firefox Nightly, don't know about stable.

Maybe! Folders in Zen let you group, label and collapse tabs, so if that's the same thing then yes.

I found it too buggy in my usage, it just doesn't compare to the polish in Zen or the other forks. Better just to use the horizontal tabs IMO.