Wonderful! Rated 4 out of 5 stars
Helps Organizing the tabs clearly.
Just a bit of nitpicking:
(1) When I open links with "Super Drag-and-Go" the subtree structure isn't maintained (I get a new "parent" tab instead of a "child" tab).
Super Drag-and-Go add-on: https://addons.mozilla.org/en-US/firefox/addon/137
(2) It might be yet more perfect if the child tabs assumed the original color of the parent using "colorful tabs" extension:
GREAT work. Thanks so much :)