What errors are you getting?

Start your Computer in safe mode with networking.

Starting The Computer In Safe Mode;
Free Online Encyclopedia

Uninstall Firefox from your computer {web link}
DO NOT remove your profiles. After, reboot the computer. Then run a registry scanner. After,

https://support.mozilla.org/en-US/kb/update-firefox-latest-version

Thank you very much for your reply Fred, but unfortunately it doesn't help. The System Restore put everything back the way it was* but, using Internet Explorer, I cannot download new copy of FF or even remove the existing version. When I click [Remove] from the Add/Remove programs section of control panel, nothing at all happens.

• The original problem was caused by a commercial USB thumb drive that was supposed to contain photos and video made of me with a GoPro camera. I could see not photos (in the directory) and clicking on the mp-4 video totally messed up the display -- icons 4X size, weird colors, changed wallpaper on desktop. That's why I did the System Restore.

Thank you again for trying to help. Bruce

FredMcD said

What errors are you getting? Start your Computer in safe mode with networking. Starting The Computer In Safe Mode;
Free Online Encyclopedia Uninstall Firefox from your computer {web link}
DO NOT remove your profiles. After, reboot the computer. Then run a registry scanner. After, https://support.mozilla.org/en-US/kb/update-firefox-latest-version

http://www.mattsilverman.com/2009/06/how-to-download-firefox-without-a-web-browser.html

also,

Using another computer, Download Download Firefox Full Version For All languages And Systems {web link} onto a thumb drive. After, plug the thumb into your system.