osx 10.13.6 on a 15-inch 2016 macbook pro with two external 4k monitors. firefox nightly 63.0a1 (2018-08-08) my primary screen's resolution is set to "default for display"; ie hidpi 1920x1080 my second 4k monitor's default resolution is 3840x2160, so i have that set to "scaled" down to 1920x1080. on my primary screen all is well. on my secondary screen the scrolling performance of firefox with webrender enabled is horrible. the experience feels like the CPUs are pegged at 100% calculating primes or something in the background; extreme yank. needless to say the CPUs are sitting relatively idle; the yanky experience is consistent regardless of the system's load, and other applications on the same monitor have buttery smooth scrolling, including firefox without webrender enabled.

Do you have a discrete GPU as well as an integrated one? I assume scrolling is bad regardless of the content?

yup; Radeon Pro 460 4096 MB GPU (which osx is telling me it's using) in addition to the Intel HD Graphics 530. yes, scrolling is bad regardless of content.