It looks like a bug nearly half a decade old has finally been fixed by Microsoft and Mozilla. The issue was related to Windows Defender and its Antimalware…
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Microsoft and Mozilla have managed to fix a five year old bug in Windows Defender which was causing relatively high CPU usage on Firefox compared to other browsers like Chrome and Edge.
For more than five years, the troublesome security protection provided by Microsoft Defender was negatively affecting Firefox users during their web browsing sessions. The Antimalware Service Executable.