Let’s do a full clean re-install;

Download Firefox For All languages And Systems {web link}

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 any profile folders.

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

You can run the MMS installer via this file in the Firefox program folder.

• maintenanceservice_installer.exe

Thank you to FredMcD for your time and response. However, it appears there is no need to go through the time and complexity of your proposed solution. Please refer to the response from cor-el which has apparently restored my PC to what I have been expecting. I would add the following clarification to cor-el's solution, to assist future users. My Win 10 PC has a C:\Program Files folder and also a C:\Program Files(x86) folder. The requisite / needed folder as suggested by cor-el was found in my C:\Program Files\Mozilla Firefox folder. When the installer file was run, it happened quickly and almost instantaneously. Although it appeared as if nothing happened, it did update my MMS to the current version One further word of caution - there are other “maintenanceservice” files in both Program Files and Program Files(x86), but these are not to be clicked on or need to be run. There is only one “installer” file.

The MMS service (maintenanceservice.exe) is installed in its own folder (%PROGRAMFILES%\Mozilla Maintenance Service). Once you have grated UAC access for this application when you first installed it, no further approval is necessary.

• https://support.mozilla.org/en-US/kb/what-mozilla-maintenance-service