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