From the 07-Apr-2023 ghacks.net article Microsoft Fixes 5 Year Old Windows Defender Bug That Affected Firefox's Performance :
Microsoft has fixed a bug in Windows Defender that was leading to high CPU usage when Firefox was open. It only took the company 5 years to fix the issue.
When Firefox was running, Windows Defender's Antimalware Service Executable would act up, causing its CPU Usage to rise significantly. Many users said that the performance was so bad that their PCs would lag when using the browser. Some people had compared the performance with other browsers such as Chrome and Edge, and found that it didn't affect them, the bug was limited to Firefox....You don't need to do anything, the bug has been patched in the March 2023 update that was released on April 4th. It bumps the app's version number to 4.18.2302.x, and patches the Engine to version 1.1.20200.4....
... It is worth noting that this patch only applies to Windows Defender, and not other antivirus programs, but some users have reported a similar issue with other security software such as Norton Antivirus. Mozilla is already working on more improvements to patch the issue with other security applications. (Refer: 1 and 2)
Kudos to Alex5723 for posting about this ghacks.net article today <here> in the AskWoody forum.
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Dell Inspiron 5584 * 64-bit Win 10 Pro v22H2 build 19045.2728 * Firefox v111.0.1 * Microsoft Defender v4.18.2302.7-1.1.20200.4 * Malwarebytes Premium v4.5.25.256-1.0.1957 * Macrium Reflect Free v8.0.7279