Have you noticed that your PC is slower when browsing Firefox? The reason for this is a bug in Microsoft Defender, which has finally been fixed.

The problem is not from yesterday: The first report came in May 2018 on the Firefox Bug Channel. The browser increased the CPU usage of Microsoft Defender, which negatively impacted the browser’s performance. Curiously, this bug also turned out to be exclusive to Firefox, Defender leaves the Chromium browsers Chrome and Edge alone.

Mozilla developers spent years trying to figure out the cause of the problem and only a month ago concluded that Microsoft Defender’s built-in anti-malware protection was the culprit. The bug causes more events to be fired that cause Microsoft Defender to spring into action. As a result, Defender calls VirtualProtect in Firefox up to five times more often than any other browser and increases CPU usage by up to 75 percent.

A solution after five years

Mozilla reported the problem to Microsoft and the two companies worked together to find a solution. It was introduced by Microsoft on April 4th, almost five years after the first report. Normally users don’t need to do anything and Defender should update automatically. This update was even rolled out for Windows 7 and 8.1, although these Windows versions are no longer supported.

You can check this yourself by looking for the folder C:\ProgramData\Microsoft\Windows Defender\Definitions Updates and then mpengine.dll right click. The correct version is 1.1.20200.4. The Defender app must be in version 4.18.2302.x sit.

The fact that the problems at Mozilla have dragged on for so long has not harmed Microsoft. Edge has been catching up with Firefox since 2018. In 2018 Firefox was still the third largest browser and Edge was out of the question. Today, Edge is number three with a market share of almost five percent; Firefox fell below three percent (via Statista).