I recently force-enabled multiprocess FF on a machine that usually crawls due to memory starvation, and it's felt much snappier than before, though heavy single page apps are still not its forte.
If I ever switch, it'll probably be to Chromium. Safari is fast, but the URL bar is unusable and it's not very customizable and the Dev tools are meh. I'm avoiding Chrome for privacy reasons and Chromium for battery life reasons (and also because of 12 years of inertia and muscle memory).
I've looked at Opera and Vivaldi and the like, but their features seem to be aimed at power users of social media, not at developers or someone looking for an old-school browser experience.
These days there isnt much choice though, its Gecko, Blink or Trident (or is it EdgeHTML now?). All the apparent alternatives are pretty much shells.
It doesn't have the treestyle tabs though. Actually I never looked into it, sounds interesting. Hm.