Microsoft has finally fixed a bug in Windows Defender that caused high CPU usage when used with Firefox, the Firefox browser. This bug has existed for 5 years and has only been fixed now.
Windows Defender enabled the Antimalware Service Executable feature when users enabled Firefox browser to surf the web, causing a significant increase in CPU usage.
The bug was first reported in May 2018 and affected both Win10 and Win11 platforms, according to Bugzilla.
Mozilla engineers said the problem is caused by Msmpeng.exe (Microsoft malware protection engine) in Defender. The Firefox browser has a 5x higher CPU usage compared to Chromium-based browsers like Chrome and Edge.