Step one: Reboot the computer. This should release anything locked/not working. 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.