Tree Style Tabs should continue to work. It's a very popular extension. Mozilla should probably include a disclaimer about Tree Style Tabs still working with an

Tree Style Tabs should continue to work. It's a very popular extension. Mozilla should probably include a disclaimer about Tree Style Tabs still working with any blog post about WebExtensions. :) Mozilla is now beta testing a prototype of native Firefox vertical tabs, though it is not as advanced as Tree Style Tabs: https://testpilot.firefox.com/experiments/tab-center https://testpilot.firefox.com/experiments/tab-center