There is a known issue in Firefox version 49 that causes images and text to display as smeared or streaky lines or tears while scrolling when used with certain older graphics cards. I understand you are using 76.0, which is the latest so shoud not retain the same. There can be a misconfig causing the same, so you may want to try this solution, please use the steps as in the link: https://support.mozilla.org/en-US/kb/streaks-smeared-lines-or-blurry-web-pages-scrolling

Please note, it may ask to choose the datatype at one point in the steps, then go with Boolean.

You can try to disable hardware acceleration in Firefox.

• Options/Preferences -> General: Performance
  remove checkmark: [ ] "Use recommended performance settings"
  remove checkmark: [ ] "Use hardware acceleration when available"
• https://support.mozilla.org/en-US/kb/performance-settings

Close and restart Firefox after modifying the setting for changes to take effect.

• https://support.mozilla.org/en-US/kb/troubleshoot-extensions-themes-to-fix-problems

You can check if there is an update for your graphics display driver and check for hardware acceleration related issues.

• https://support.mozilla.org/en-US/kb/upgrade-graphics-drivers-use-hardware-acceleration

This appears to be a problem with the computer. Occurring with Edge as well and on windows 10 start screen. Brand new just out of the box Lenovo P53 with NVIDIA T2000 Graphics card.

Are there updates available for the graphics display driver ?

Boot the computer in Windows Safe mode with network support to see if that has effect in case security software is causing problems.

• https://www.bleepingcomputer.com/tutorials/how-to-start-windows-in-safe-mode/
• https://www.7tutorials.com/4-ways-boot-safe-mode-windows-10