[Affected versions]:

• Fx 71.0.1
• Fx 71.0b7
• Fx 72.0a1 (2019-11-07)

[Affected platforms]:

• Windows 7 x 64

[Steps to reproduce]:

• Open https://bug1190112.bmoattachments.org/attachment.cgi?id=8642106
• Scroll the entire content and observe the scrollbars inside the page

[Expected result]:

• The scrollbars don’t flicker during the scrolling action

[Actual result]:

• The scrollbars flicker during the scrolling action

[Regression range]:

• Not a regression, I can reproduce this issue way back to Fx 30.0a1 (march 2014).

[Additional notes]:

• About:support content https://pastebin.com/ThXspxP6
• Setting layout.css.scroll-anchoring.enabled on false makes no difference
• Scrolling inside any of the test cases provided inside the page has no flickering effect on the long page scrollbar
• I could see the same issue, also when involves more than one scrollbar with the following
  test case: https://bug1412272.bmoattachments.org/attachment.cgi?id=8925062 (only on the first horizontal scrollbar)
• Not reproducible with Chrome on the same machine
• Not reproducible on Win 7 machines with ATI Radeon 3000 and NVIDIA GeForce 21
• Attached a screencast (couldn’t catch the problem with a screen recorder), unfortunately the issue is hard to observe with this video, but it is very prominent on the actual screen

I wasn't able to reproduce this, but based on the symptoms and the fact that it only occurs with some video cards, this is most likely a graphics issue.