tech support 101

Close browsers, Uninstall Flash, reboot, reinstall Flash and test.

If that doesn't work, then Close browsers, Uninstall Flash, Uninstall Firefox, reboot, reinstall Firefox, reinstall Flash and test.

If THAT doesn't work, do all of that, but after rebooting in the middle, delete their C:\Program Files\ folders and use the "Registry scan/fix option" in CCleaner (search Google, it's free) to clean out orphaned entries from the registry, reboot and reinstall.