Earlier quoted context omitted.
https://github.com/piroor/treestyletab/issues/1224 Seems they have all the APIs needed for it at the very least approved. It's not like Mozilla just said screw everyone... Let's become chrome. They are putting in the effort to make web extensions viable for even the more advanced uses.
They are making to where a version of TreeTabs are possible, this new API will enable functionality like in Chrome (see my other comment) but it will not be the same. if I wanted Chrome style Vertical tabs I would have moved to chrome years ago
Unrelated, but the fact that the developer doesn't get the extension AMO approved and uses eval() all over the place should give some pause as well.