"by Piro (piro_or) (Developer) on September 26, 2017 · permalink
Due to unavoidable reasons, TST 2.0 is strongly depends on new WebExtensions APIs introduced at Fx57, so it can't become compatible to older versions of Firefox. Please keep using TST 0.19.x for a while, until Fx57 is released."

Nope. A few days ago the tree structure stopped working in my Firefox 55, with TST version 0.19.2017090601. Now I just have a vertical column of tabs. The whole point of using TST is to be able to arrange tabs into trees, right? Like others are commenting here, TST is a "must have" extension for me. What's up?