>with Firefox 4 until they removed it because "nobody used it".

Tab Groups remains a good idea on paper but no one has done a useful implementation of it. I remember during the Firefox 4 Tab Group era I purposed that Tabs from site ( Say HN ) and any Tabs opened from that site, for example I have another tab from this submitted links bugzilla.mozilla.org. All To be moved together into a tab group instead of just moving site to a tab group.

It was rejected for whatever reason.