The trick to using Firefox on mac like 5 years ago was to use the developer edition. For some reason it was way faster than standard Firefox. I don't know if that's still the case, but I stick to it for good luck.
For a few years, DevEdition had multiple content processes enabled (codename "electrolysis", aka e10s), while it was still off in release builds. That made a huge, huge difference to responsiveness and resilience.
