Firefox fonts "broken" but ONLY in Secondary display (duplicate main s

I had originally thought I had a problem with Firefox, Firefox fonts suddenly "broken", but it turns out that the problem shows up only on a secondary display but not on the primary display, which is a Lenovo 730 laptop. THERE IS NO ISSUE WITH WINDOWS OR ANY OTHER APPLICATION.

Details:
System is Lenovo730, 8550U CPU, 16 GB RAM, Intel UHD 620. Secondary display is Samsung 17", installed as a generic PnP monitor. Windows 10 64 Home with all patches applied. Firefox is 86.0.1. If you read through the earlier thread above, you will see that I used DDU to clean out the old GPU driver before installing the newest GPU driver which I downloaded from the Lenovo website.

If I look at the laptop screen, all the text in Firefox tabs looks normal. However, on the secondary display, the text looks like a lot of the pixels are missing or maybe they are light gray. My wife describes the appearance as "broken fonts."

Because I just looked at the laptop primary screen today, I had assumed that I had a Firefox issue. Now it's clear that Firefox works fine, but somehow there is an issue with displaying Firefox on the secondary display.

If it's only on the secondary screen, I would check the cable and the secondary screen.
If possible, swap the cable, and if that doesn't help... swap the screen.

Secondary display is Samsung 17"

the secondary monitor is very old and doesn't support the resolution you have set for your laptop.

spunk said:

the secondary monitor is very old and doesn't support the resolution you have set for your laptop.

Actually the secondary monitor does work OK for all applications except the Firefox browser. And until about a week ago, it worked fine for Firefox, and was working fine for about 18 months since my wife got the Lenovo 730.

Do a System Restore to the time when it was working. You can drag the Firefox window to the external monitor and change the Font in Firefox to one that works for your monitor.
Or you Roll back the video driver to the previous one that was working, if it got updated by Windows Updates. try a different browser to test.