This addon finally took me to the latest release of Firefox, the 60th! I had been sticking with a old one, Firefox 56, because no available replacements for tab-grouping addons seemed to exist. I am very very grateful that this idea was re-implemented in the newest version of Firefox and would push the developer(s) to improve and refine it more.

Improvements and fixes suggestions as to 25/05/2018:
1. Tab hiding doesn't seem to work quite well. I have switched between two groups using the CTRL+w key and their tabs have been shuffled all together, that is a merge was formed.
2. The Panorama view could be implemented without opening a new tab. That would feel much more tidy.

Addendum #1:
I found a fix to point 1.! Open the preferences page at about:config and look for the extensions.webextensions.tabhide.enabled configuration. Set it to true and let the plugin work as it should.

Addendum #2:
Would you mind adding an auto-backup feature? Too many a time have I crashed my browser and lost all of my tabs. Having no backup at hand, and no way of restoring my previous windows session, I had to start over collecting and organizing my tabs.