Microsoft finally fixes 5 year old Windows Defender high CPU bug on Mozilla Firefox

It looks like a bug nearly half a decade old has finally been fixed by Microsoft and Mozilla. The issue was related to Windows Defender and its Antimalware Service Executable (MsMpEng.exe) service which would lead to high CPU usage on Mozilla Firefox. The resource usage was noticeably higher compared to Google Chrome and Microsoft Edge. For example, the image below (taken at the time of initial reporting of the bug) shows average CPU usage when reloading YouTube six times. As you can see, the spikes were clearly higher on Firefox.

The bug was recently resolved by the efforts of Microsoft and the Mozilla development team. Yannis Juglaret, a Firefox developer confirmed this around three weeks ago:

According to Microsoft, this will be deployed to all users as part of regular definition updates, which are packaged independently from OS updates. This includes even Windows 7 and 8.1 users, even though these platforms should not have had the performance issue with Firefox in the first place because the ETW events that cause it do not exist on these older versions of Windows.

Later on, Yannis Juglaret added that the recent Microsoft Defender March-2023 definition update (Platform: 4.18.2302.x | Engine: 1.1.20200.4) fixed the issue:

mpengine.dll version 1.1.20200.4 was released on April 4, so the fix should be available for everybody now.

Here are details for the Defender update:

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

• Security intelligence update version: 1.381.61.0

• Release date: April 4, 2023 (Engine) / April 11, 2023 (Platform)

• Platform: 4.18.2302.x

• Engine: 1.1.20200.4

Interestingly, it has also been found that there is further scope of improvements to the processor usage in Firefox when compared to Chrome. Perhaps we will see such performance improvements in upcoming browser updates and it won't just be exclusive to Microsoft Defender alone.

Microsoft finally fixes 5 year old Windows Defender high CPU bug on Mozilla Firefox