You can check for issues caused by a corrupted or incomplete Visual C++ installation that is missing runtime components (Redistributable Packages) required by Firefox, especially VCRUNTIME140_1.dll. See "Visual Studio 2015, 2017 and 2019": *https://docs.microsoft.com/en-US/cpp/windows/latest-supported-vc-redist *https://aka.ms/vs/16/release/vc_redist.x64.exe *https://aka.ms/vs/16/release/vc_redist.x86.exe You may also have to install the Visual Studio 2013 Redistributable Package. You need to reboot the computer. ---- It is possible that your firewall or other security software blocks or restricts Firefox without informing you, possibly after detecting changes (update) to the Firefox application. Remove all rules for Firefox from the permissions list in the firewall and let your firewall ask again for permission to get full, unrestricted, access for Firefox and the plugin-container process and the updater process. See: *https://support.mozilla.org/en-US/kb/server-not-found-connection-problem *https://support.mozilla.org/en-US/kb/Firewalls *https://support.mozilla.org/en-US/kb/fix-problems-connecting-websites-after-updating