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A performance-sapping conflict between Mozilla Firefox and Microsoft Defender was first discussed on Bugzilla half a decade ago.
However, Firefox users can now rejoice, as Mozilla devs and Microsoft worked together to release an update to MsMpEng.exe (a core process of Windows Defender), which is currently being rolled out
/nosanitize
Microsoft acknowledged a problem with MsMpEng.exe using too much CPU time when Windows Defender's real-time Protection feature is spurred into action.
This change has helped cut the CPU usage observed by Firefox users significantly
the Firefox app was particularly hard hit by the Microsoft bug, as the browser is said to generate up to 7x more Event Tracing for Windows (ETW) events compared to competitors (Edge, Chrome etc).
https://www.tomshardware.com/news/microsoft-defender-mozilla-firefox-bug