More details from OP: apparently I still have the prior FF ver 56.0.2 on my computer. There is a message saying the download of ver58 failed due to an Internet connection problem. Can I uninstall FF to fix the problem? Preferably would like to go back to ver 56.0.2 since everything (including Yahoo mail) works with that version.

Do you still have access to the web on the 'sick' computer? If not, you will have to download the full installer using another computer and saving the file to a thumb drive.

As to Windows Internet Explorer; Perform a System File Check to fix corrupted system files

Win 7, 8, 10, Vista: https://support.microsoft.com/en-us/kb/929833 Win XP: https://www.microsoft.com/resources/documentation/windows/xp/all/proddocs/en-us/system_file_checker.mspx?mfr=true

Wiki - Windows 10 System File Check http://answers.microsoft.com/en-us/windows/wiki/windows_10-update/system-file-check-sfc-scan-and-repair-system-files/bc609315-da1f-4775-812c-695b60477a93

Let’s do a full clean re-install;

Download Firefox For All languages And Systems {web link}

or

Install Older Version Of Firefox {web link} Be sure to read everything here.

Save the file. Then Close Firefox.

Using your file browser, open the Programs Folder on your computer.

Windows: C:\Program Files C:\Program Files (x86)

Mac: Open the "Applications" folder. https://support.mozilla.org/en-US/kb/how-download-and-install-firefox-mac

Linux: Check your user manual. If you installed Firefox with the distro-based package manager, you should use the same way to uninstall it. See Install Firefox on Linux; https://support.mozilla.org/en-US/kb/install-firefox-linux

If you downloaded and installed the binary package from the Firefox download page, simply remove the folder Firefox in your home directory. http://www.mozilla.org/firefox#desktop ++++++++++++++++++++++++++++ Look for, and remove any Mozilla or Firefox program folders. Do not remove the Mozilla Thunderbird folder if there is one.

Do Not remove the profiles folder.

After rebooting the computer, run a registry scanner if you have one. Then run the installer. +++++++++++++++++++++++++++ If there is a problem, start your Computer in safe mode and try again.

http://encyclopedia2.thefreedictionary.com/Linux+Safe+Mode This shows how to Start all Computers in Safe Mode; Free Online Encyclopedia

What security software do you have?

See also;

• bug 1433065 - Firefox 58 is not loading any pages (including about: pages)

(please do not comment in bug reports
https://bugzilla.mozilla.org/page.cgi?id=etiquette.html)

Update from the OP: while trying to locate another computer to download the full installer with plans of doing a fresh install, and having done nothing to the problem computer or Firefox (other than running virus and malware checks- none found), the problems I was having miraculously disappeared! Since ver 58 failed to download (interrupted by an internet connection issue during download), I still have Firefox 56.0.2 64 bit. Would love to know how the issue fixed itself, but just happy to be up and running again!