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. Troubled users should notice a significant improvement after the update, with "a ~75% improvement in CPU usage from MsMpEng.exe when browsing with Firefox," according to senior Mozilla software engineer Yannis Juglaret.
So, what has been happening, and why was there an awful interaction between Firefox and Microsoft Defender? 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.
