I used to use Firefox for Tree Style Tabs and Pentadactyl, but I recently switched over to Qutebrowser[0] and I'm loving it. It has Pentadactyl-inspired keybinds, vertical tabs (no tree-style just yet, though), and it's _way_ snappier than FF. It's refreshing to use after all the bloat that Firefox has acquired and the slow-to-nonexistent development of Pentadactyl. (Not to mention the existential crisis of the FF WebExtensions switch.)
There are some sore points revolving around the use of QtWebKit engine, but they're mostly workable. I set the user-agent to either FF or Chrome to get some sites to not redirect me to their compatibility site, and YouTube support isn't great (no fullscreen). I just set up a keybind to send the current url to youtube-dl/MPC-HC and it works fine.