Awesome add on ! Seems to be a great successor for tabkit 2 !

Some ideas and remarks :

Color tabs when a child tab is opened , I mean that each tree could be colored in a different color : it's very useful ! (a click on a link opens a new tab in the same color of the current parent tab, the main parent tab is colored only when a child tab is opened) .

Highlight tabs in a special color for links opened in a new tab but the tab haven't been opened/clicked/viewed.

You could also add a checkbox for "mousewheel scroll on the last tab allow to return to the first one".

Shift + mouse scrool to switch between groups

A keyboard shortcut to close a tree could be nice too ;)

Add a tutorial on the add-on web page to explain how to remove the original horizontal tab bar from firefox.

Thanks for the work on this add-on !