This is very nice. I have always _wanted_ to use Firefox because a web built much for the community is best used on a browser built by the community with no corporate influences such as trying to push their own "web platform". It's just that it has felt far too bulky compared to Chrome.
But yeah, this is quite workable, and Quantum WebRender hasn't even landed yet. By Firefox 60 we should have even more substantial (i.e. noticeable) improvements.
It's scrolling very smoothly, and the browser rendering sites doesn't seem to impact typing into the adress bar much. It elegantly passes my personal little stress test with multiple Periscope tabs running at once, something that near killed Firefox 56. Honestly it doesn't even really flinch, and that's on an aging laptop, but an 8 core at that, so I guess the Quantum components so far in Firefox are happy about this.