Firefox comes in two or more folders on all computers. They are;

Maintenance: (Programs Folder) <Windows Only> Firefox itself: (Programs Folder) And at least one folder in the profile of each user on the computer.

If you remove the Firefox folder, the user folders would not be affected.

Firefox v55 has been having problems with graphics drivers and protection programs.

Check out Firefox v56 beta, some of the issues with Intel drivers have been fixed. https://www.mozilla.org/en-US/firefox/beta/all/

Thank you, Fred. Now that I know this is not a local problem, I think I'll wait for the fix from Mozilla. My efforts at chasing computer issues sometimes result in unintended consequences!

You can try to disable hardware acceleration in Firefox.

• Options/Preferences -> General: Performance
  remove checkmark: [ ] "Use recommended performance settings"
  "Use hardware acceleration when available"

You need to close and restart Firefox after toggling this setting.

• 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

Thanks for that info. Since I posted the original question I have found that Chrome stutters on the same streamed videos as FF, so it is likely a website problem. But I'm glad to have the info on reinstalling FF for general reference.