Works well and has a decent feature set. Possibility to change colors and plenty context menu options. Has a few bugs though. Some of the sidebar icons don't work. Unfortunately there is no way to hide the default Firefox tab bar until the new web extension API allows something like that.

I would also appreciate it if tabs opened from links in the active tab were treated as children of the active tab. Currently it is possible to attach all new tabs as children of the active tab, but not those opened from links specifically.