The best tree-tabs add-on. Long time user (~5 years) and I like the migration to the web-ext. The only ask for firefox developers is to put an API to hide the old tabs bar so that it can be used by add-ons like TST. Currently I am hiding the tab-bar from the userChrome.css.

Shameless plug: I have written the load-progress-bar add-on to give me a little better idea about the page load progress that I am missing in the new FF57 and I find useful in combination with TST