[enturn]

I've been using Waterfox since Mozilla changed their extension format and it's been incredibly stable. I used it for the Tree Style Tab extension. There's probably less reason to use it just for that anymore but vertical tabs in Firefox still isn't the best experience. I use Firefox with Tree Tabs at work with the extra user UI modification required to hide the top tabs and the sidebar headings. Only allowing one sidebar means when I accidentally open the bookmarks I need to mess around to get my tabs back.

[sciurus]

No nee to switch to a fork for that. Tree Style Tab 2.x supports Firefox even after the switch to webextensions.

(Disclaimer: I work for Mozilla but not directly on the browser)