It took Microsoft and Mozilla about 5 years to fix a long-known problem that caused excessive CPU usage in Windows in some cases.
The problem was related to Windows Defender and its Antimalware Service Executable (MsMpEng.exe), which actively loaded the CPU when the Mozilla Firefox browser was running. Resource usage was noticeably higher compared to Google Chrome and Microsoft Edge. For example, the image below shows the average CPU usage when YouTube is reloaded six times. You can clearly see that the peaks were clearly higher in Firefox.
The bug was only recently fixed by a joint effort between Microsoft and the Mozilla development team. Firefox developer Yannis Juglaret confirmed this about three weeks ago.
According to Microsoft, the patch was planned to be rolled out to all users as part of regular Windows Defender definition updates, which are packaged independently of OS updates. The update will even affect Windows 7 and 8.1 users, even though there shouldn’t have been any Firefox performance issues on those platforms because the ETWs that trigger their events don’t exist on those older versions of Windows.
Yannis Juglaret later added that the recent March 2023 Microsoft Defender definitions update (platform: 4.18.2302.x | kernel: 1.1.20200.4) finally fixed the problem. The corresponding version of mpengine.dll 1.1.20200.4 was released on April 4th, so the fix should be available to all users by now.
Interestingly, in the process of working on this problem, Firefox developers discovered several additional areas for improvements that will reduce the consumption of processor resources when the browser works together with other antiviruses.
Source: neowin
