Microsoft fixed the Windows Defender bug affecting Firefox’s performance. The bug, which has been around since May 2018, affects Windows 10 and 11 users, but the update was also released for Windows 7 and 8.When Firefox is running, Windows Defender’s Msmpeng.exe file is known to run, which increases CPU usage. According to research, the problem is caused by Windows Defender’s real-time protection; CPU usage increases because VirtualProtect runs multiple times. Disabling JIT alleviates the problem, but does not completely solve the CPU usage problem.
Reduced Firefox’s CPU usage
Microsoft fixed the bug by releasing a beta version of Defender. CPU usage decreased by about 75%. The bug was also fixed with the March 2023 update released on April 4th. Windows Defender version number should be 4.18.2302.x, engine version should be 1.1.20200.4. The bug fix was released for Windows Defender, but users are reporting similar issues with various antivirus programs. Mozilla is also working on solving this problem.
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