The problem of high consumption of processor resources by the Windows Defender when the Firefox browser is running has been known for five years — and finally Microsoft decided to deal with it.
The problem was related to the Antimalware Service Executable (MsMpEng.exe) process, which used significantly more CPU resources when Firefox was running on the system than with other browsers such as Chrome or Edge.
It seems that Microsoft has finally learned that Firefox still has an audience, and a joint effort with the browser’s development team has fixed the bug: it was reported three weeks ago by one of the developers, Yannis Juglaret. He later added that in April Microsoft released Windows Defender update that finally fixes this problem.
As he explained, the fix is that the real-time protection feature of Windows Defender will now consume much less CPU resources when monitoring the behavior of any application (not just Firefox). For Firefox, this is especially important because the browser relies heavily on VirtualProtect, which is controlled by the MsMpEng.exe process. As the Mozilla team expects, this process will now consume 75% less resources when tracking Firefox behavior.