"The solution was delivered to the stable channel with updated Defender antimalware definitions on April 4 (mpengine.dll version 1.1.20200.4) and the bug was finally closed. Mozilla developers said that the Defender update would provide a massive ~75% improvement in CPU usage while browsing the web with Firefox".
Source: https://www.techspot.com/news/98255-five-year-old-windows-defender-bug-killing-firefox.html?
Quote~75% improvement in CPU usage
That is massive!
That is amazing. That bug made me ditch FireFox. I remember spending many hours trying to find a way to fix FF excessive CPU useage that would build and build until it froze my processor. And here it was a MicroSoft bug all this time.