Microsoft Addresses Long-Standing Bug Impacting Firefox Performance After 5 Years
After 5 years of waiting, Microsoft has finally fixed a bug that directly affects Firefox on devices with Windows operating system. This bug is directly related to Windows Defender and causes high CPU consumption when Mozilla browser is running. The truth is that this bug that has existed since May 2018 does not only affect devices with Windows 10, since the latest version of its operating system, Windows 11, is also affected by it.
The problem arose when Firefox was running, causing the Windows Defenders Antimalware Service Executable (Msmpeng.exe) to act up, causing a significant increase in CPU utilization. Users reported that their PCs were slow when using this browser, and the problem was limited to Firefox as it did not affect other browsers like Chrome and Edge.
Investigations revealed that Windows Defender’s real-time protection was the root cause of this bug, invoking VirtualProtect multiple times. Mozilla engineers worked with the Microsoft team to resolve the issue, concluding that calls to VirtualProtect were abnormally high, leading to the performance issue. Disabling JIT in about:config mitigated the issue, but did not completely resolve the CPU usage issue.
The fix is also being rolled out to Windows 7 and 8.1 users even though they were not affected by this issue.
To check if you have the latest version of the DLL, go to C:\ProgramData\Microsoft\Windows Defender\Definition Updates, open the folder with a long alphanumeric name, right-click on mpengine.dll, select Properties, switch to the Details Tab and check the product version, which should be 1.1.20200.4. While this patch only applies to Windows Defender, some users have reported similar issues with other antivirus programs such as Norton Antivirus.
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