Some problems occur when your Internet security program was set to trust the previous version of Firefox, but no longer recognizes your updated version as trusted. Now how to fix the problem: To allow Firefox to connect to the Internet again; * Make sure your Internet security software is up-to-date (i.e. you are running the latest version). * Remove Firefox from your program's list of trusted or recognized programs. For detailed instructions, see '''[https://support.mozilla.org/en-US/kb/configure-firewalls-so-firefox-can-access-internet Configure firewalls so that Firefox can access the Internet.]''' {web link} ------ 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. Install both 32-bit and 64-bit "Visual Studio 2015, 2017 and 2019" downloads, (x86: vc_redist.x86.exe and x64: vc_redist.x64.exe) from https://support.microsoft.com/help/2977003/the-latest-supported-visual-c-downloads You may not need the latter (32-bit version) if you use 64-bit Firefox. You may have to reboot the computer. ++++++++++++++++++++++++++++++++++++++++++++++ In another thread, a user reported that a recent update of Malwarebytes Anti-Exploit (version 415) may be causing symptoms similar to the VC Runtime problem. If you have MBAE, it should have a setting to exempt Firefox. You could test whether that makes any difference.