Let’s do a full clean re-install;

Note: Firefox comes in three or more folders on all computers. They are;

Maintenance: (Programs Folder) <Windows Only> Firefox itself: (Programs Folder) And two folders in the profile of each user on the computer for each Firefox profile for that user.

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

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) Note: Check Both Folders

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.

How to Start all Computers in Safe Mode; {web link} Free Online Encyclopedia

I uninstalled Libre Office and it apparently uninstalled the VCRUNTIME140_1.dll file s part of its uninstall process. From this point Firefox would open but would not connect to the Internet, either by clicking one of the links in the toolbar or typing the address directly. I have tried all the solutions I can find on the support pages, ranging from attempting to copy the file from another location to re-installing Visual C++ from the Microsoft website, both the 64 bit and the x86 versions. Having found and installed the latest version of Visual C++ I now get the error message "The procedure entry point _CxxFramehandler4 could not be located in the dynamic link library C\WINDOWS\SYSTEM32\MSVCP140.dll" at the edn of the installation process, and of course Firefox has not installed. Any suggestions for a way forward?

(For the record) Related bug report: Bug 1624546 Opened 2020-03-24 05:18 EDT Updated 2021-03-08 15:36 EST Include vcruntime140_1.dll for VS2019 16.5 x64 from the redist directory in the package