Nice and promising addon in the style of "tree style tab" and other tab managers.

But please,
a) add a "F1" feature like TST for opening/closing the list of tabs in the sidebar,
b) add a feature to "size" the list according to user's eyes ..., (tabs height in the list)
c) add an "indent" feature in the list to show relations between tabs (without the parent-child-sibling thing that never works as it should).

In brief, keep it simple and functional !
I like your "forecasts" in the addon future as Firefox v57 will evolve to v. 58 ... So, one knows where things are going ...
;-)