This issue is quite long, so there are both a short version and a longer one.
Setting these configurations fix a graphical glitch which often happens on Firefox while scrolling :
gfx.xrender.enabled to true;
- layers.acceleration.force-enabled to true.
This bug happens (at least) on computers running Solus with an Intel CPU with Intel Integrated Graphics.
I installed Solus on my laptop and on my sister's one about one year ago (they are both Lenovo Thinkpad, Intel processor and using Intel Integrated Graphics). Shortly after the installation, I noticed there was some glitches on firefox while scrolling even when the package xorg-driver-video-intel was installed.
I kept using Firefox because I didn't want to install Chrome, and unsuccessfully tried to get rid of this scrolling bug.
Last week, a couple of tweets (I and II) suggested to set to true the values of
gfx.xrender and layer.acceleration.force-enabled. Turns out this solved my initial problem!
I think these two configurations should be set to true by default since they make Firefox run better out of the box.
Have a nice day,
EDIT 2018 11 14: turns out gfx.xrender has nothing to do with graphical scrolling, so it is not necessary to enable it.