Steps to Reproduce: Login to facebook messenger, and find a conversation with history. Scroll up a little bit (there may be hicups loading archived content, this isn't what I'm worried about), then begin to scoll down.
Scrolling down is smooth, as if all content is already rendered and ready.
Scrolling down is a herky-jerky disaster as the page hiccups, stopping and starting scolling all the time.
While I can say this didn't used to be like this in Firefox, I have used mozregression to test Firefox 60, and it shows very similar behaviour, so it's not a behaviour change in Firefox methinks.
(Sorry, I wasn't able to produce a video without personal info that shows the issue, and subset videos aren't great either; ni? me and I can provide privately a video of the situation)
Profile URL: https://perfht.ml/2OT63WO
Basic systems configuration:
OS version: OS/X 10.15.1
Radeon Pro 560X 4 GB
Intel UHD Graphics 630 1536 MB
Number of cores: 6
Amount of memory (RAM): 64GB