A recent upgrade installed the new version of Firefox: 78.15.0esr.
It was released on 5 Oct 2021.

It has a very annoying behavior and I wonder if others experience
it. I have torbrowser installed. It works fine, and so I don't think
it is a problem with X window system. I purged Firefox and
reinstalled it with no custsomization, and it still showed the
behavior. So it seems part of the Firefox release itself.

The behavior is hard to describe. When I scoll text up or down by
using the arrow keys, the lines of text begin to break up and
become unreadable until they settle about one second after the
scroll ends. Actually it is thin horizontal white lines less
than a cm wide that last a second. Nowhere else do I have
this problem with scrolling.

Another symptom: when the cursor is moved to or away from the
scroll bar, the grey scrollbar is broken into a vertical set of
grey and light grey lines for about a second. When I drag the
scrollbar to scroll text, text and images are broken up by
horizontal white linees for a about a second. In a colored
image these lines can be colors from the image.

--
Haines Brown