Microsoft has finally fixed a major bug in Microsoft Defender — after a whopping five years — that led to CPU spikes when using Firefox.

The bug was filed on Mozilla’s Bugzilla tracker and listed as a “defect” from an outside application that negatively impacted Firefox performance. Defender could cause CPU usage to spike when using the open source browser, impacting performance for the browser and the entire system.

According to reports on Bugzilla, Microsoft has finally made a change that fixes the bug and brings CPU usage back down to a reasonable level.

User Yannis Juglaret outlined the improvement:

I was able to install mpengine.dll version 1.1.20200.3, which should incorporate the fix for the performance issue mentioned in comment 82 and described in comment 87. I confirm that this performance issue has correctly been addressed. Attached is a comparison of a previous recording (top) with a new recording (bottom).

The numbers suggest a ~75% improvement in CPU usage from MsMpEng.exe when browsing with Firefox (the specific number applies to browsing youtube.com on my machine), bringing it close to where CPU usage was when browsing with Chrome back in comment 78 (of course, since the performance improvement benefits all programs installed on the system, the new numbers for Chrome should be lower if measured with version 1.1.20200.3 of mpengine.dll).

Of course, one can’t help but wonder how quickly Microsoft would have fixed the bug if Defender had been impacting Edge performance as much.