FIXED: Unfortunately, the latest version (4.4.2.1, from 26 Feb 2020) contains 2 major bugs, which ruined the previously awesome add-on:

1) When I now click the STG icon in the Firefox (v73.0.1) toolbar, the add-on shows me a listing of the _tabs_ in the active group, rather than a listing of all the _groups_ that I have created.

2) The "Move tab to group" functionality (from right-click context menu) no longer works. It seems to do absolutely nothing now.

(Assuming that the above bugs will be fixed soon, I'll gladly recommend the add-on to all users!)

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EDIT (5 March 2020): Yes, the update to v4.4.2.5 indeed fixed the bugs mentioned above. Superb! Thank you Drive4ik. =) =) =)