It's always been Firefox's position that features that don't get used by very many people are better off as add-ons. That way you don't end up with a browser with 1,000 features each of which adds complexity and slows development. The official recommendation is to use this add-on, which was created from the original Firefox Panorama code. http://fasezero.com/addons/
Edit: The page that made the above recommendation https://support.mozilla.org/en-US/kb/tab-groups-removal
That's fine, until they decided to neuter their add-on API.
As long as they're committed to maintaining the feature add-ons under the new API I couldn't care less how it works under the hood.
Regarding Panorama, I was and still am a huge Panorama user and the extension they replaced it with works just as well. Stuff like that is cleaner as an extension anyway (which of course begs the question of why the hell Pocket and Hello aren't extensions).
That's nice and all, but if the people behind firefox keep on taking away the features that convince people to use it (Panorama, for starters) then these little accomplishments don't accomplish much.