Is that an extension? It doesn't seem to be built in to FF. It would be great if there was tab grouping by tags and tags showing up on tab hover over!

It's built-in, you can add tags when you create a bookmark in firefox and yep, that tab grouping feature would be nice, Chrome has tab groups and you can name so it gets close.

It appears that you can't tag an existing bookmark, but only when you create it. Seems like a hobbled feature...