50
Waterfox × TreeStyleTab: A Collaboration for the Waterfox Sidebar
(www.waterfox.net)
If it's free and open source and it's also software, it can be discussed here. Subcommunity of Technology.
This community's icon was made by Aaron Schneider, under the CC-BY-NC-SA 4.0 license.
The Waterfox blog post doesn't mention it, and you'd need to change userChrome.css to do that atm. Personally I use TreeStyleTab as my primary tab view.