Microsoft no longer sells Windows 10 licenses directly from its own site, while Windows 10 support ended in October 2025, so I started testing upgrades from the latest Windows 10 build to the latest Windows 11 build to see what might break before rolling the change out more broadly.
It did not take long to find two problems. First, Windows Update remained stuck in a Windows 10 state and would not continue updating Windows 11 properly. That issue is separate and will be covered in another article.
Second, the latest version of Mozilla Firefox would not launch and returned a VCRUNTIME140_1.dll not found error. Uninstalling and reinstalling Firefox did not fix it.
Resolution
Install the Microsoft Visual C++ Redistributable package. The missing VCRUNTIME140_1.dll file is included in the 64-bit redistributable, and installing that package has resolved this Firefox launch error for other affected users as well.
Microsoft Visual C++ Redistributable for Visual Studio 2015–2022
- x64 (64-bit):
Download vc_redist.x64.exe - x86 (32-bit):
Download vc_redist.x86.exe
In my case, installing only the x64 redistributable was enough to get Firefox working again.
The more interesting question is why the upgrade left that runtime unavailable in the first place. Mozilla’s own bug tracker has documented cases where Firefox could fail when the required Visual C++ runtime was missing or disrupted, with reinstalling the redistributable fixing the problem.