Microsoft has fixed a 5 year old problem between Firefox and Windows Defender

This is essentially an issue related to the Windows Defender tool and the Antimalware Service Executable (MsMpEng.exe), which results in unusually high CPU usage for the Firefox browser running on Windows.

In other words, this bug causes Firefox's system resource usage to be significantly higher than that of Chrome or Edge. For example, the screenshot below (taken at the time of the initial bug report) shows the average CPU usage when reloading YouTube six times. The spike can be easily seen on Firefox.

Microsoft has fixed a 5 year old problem between Firefox and Windows Defender Picture 1

After 5 years since it was first reported, this bug has finally been resolved recently thanks to the efforts of Microsoft and the Mozilla development team. Yannis Juglaret, a developer on the Firefox team confirmed this about three weeks ago:

According to Microsoft, these troubleshooting adjustments will be rolled out to all users as part of regular updates, packaged and released independently of operating system updates. Notably, the target audience includes Windows 7 and 8.1 users, although these platforms have essentially had no performance issues with Firefox in the first place because the ETW events that cause the problem don't exist. on such old versions of Windows.

Then, Yannis Juglaret added that the Microsoft Defender update recently released in March 2023 (Platform: 4.18.2302.x | Engine: 1.1.20200.4) fixed the issue:.

mpengine.dll version 1.1.20200.4 was released on April 4th, so the fix should be available to everyone now.

Here are the details of the Defender update:

March-2023 (Platform: 4.18.2302.x | Engine: 1.1.20200.4)

Security information update version: 1.381.61.1.0
Release Date: April 4, 2023 (Engine) / April 11, 2023 (Platform)
Platform: 4.18.2302.x
Engine: 1.1.20200.4

Some reports show further improvements to processor usage in Firefox when compared to Chrome once the issue is fixed. We'll probably see such performance improvements in upcoming browser updates, and it won't be exclusive to Microsoft Defender.

Marvin Fry

Update 14 April 2023

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Microsoft has fixed a 5 year old problem between Firefox and Windows Defender

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