Microsoft Defender: Firefox performance brake solved after five years

For years, Microsoft’s antivirus Defender for Windows 10 and Windows 11 was a bit overzealous when the Firefox web browser was active. The error caused a significantly increased processor load. The relevant entry, which was opened five years ago in the Mozilla bug tracker, was recently closed as “solved” thanks to a Microsoft update.

The Bugtacker entry describes the bug five years ago: “Most of the time that Firefox is active is consumed by the built-in Windows 10 Antimalware Service Executable well over 30 percent of my CPU and reads and writes random files to it Windows/Tempall with etilqs_ begin. This slows me down considerably and makes Firefox feel very slow”. Setting an exception for the Firefox process helped as a countermeasure. The file accesses therefore relate to SQLite database activities by Firefox.

Almost three weeks ago, the Firefox developers were able to test a beta version of Microsoft Defender, which apparently fixed the error. The beta engine mpengine.dll 1.1.20200.3 significantly reduces processor load by 75 percent in the Defender process when the Firefox web browser is active MsMpEng.exewhich puts it roughly on par with Google Chrome.

The updated virus scanner engine version 1.1.20200.4 is now generally available for all supported Windows versions. Microsoft’s changelog does not list the fix for Firefox users, instead classifying the update as a general “security and critical update”. The distribution usually takes place automatically via the Windows Update mechanisms.

According to the Mozilla bug tracker, “This will be made available to all users as part of the regular definition updates that are distributed independently of operating system updates. This also applies to Windows 7 and 8.1 users, although these platforms do not have the performance issue with Firefox at all shouldn’t have in the first place since the ETW events that cause it don’t exist on these older versions of Windows”.

Firefox users are now likely to experience significantly more sloppiness with the web browser on Windows if they use Microsoft Defender as virus protection.

In November last year, Microsoft missed better protection functions in Microsoft Defender in the enterprise area, for example against ransomware. However, two weeks ago, the Microsoft virus protection irritated with a false alarm in the Windows Security Center that device security was not guaranteed.


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