Windows Defender Finally Squashes Firefox Bug That Ate CPUs for 5 Years

Jonathan Smith
Apr 11, 2023
Recent findings suggest that a CPU bug affecting Windows users was actually caused by Windows Defender, rather than Firefox itself.
Windows Defender Finally Squashes Firefox Bug That Ate CPUs for 5 Years

Although Firefox has long been criticized for being a resource-intensive browser, this may not be entirely justified. Recent findings suggest that a CPU bug affecting Windows users was actually caused by Windows Defender, rather than Firefox itself. The most recent update to this widely-used security tool addresses the problem, leading to a noticeable reduction in CPU usage when running Firefox on Windows.


As per Mozilla senior software engineer Yannis Juglaret (as reported by Neowin), the source of the problem was MsMpEng.exe - a process that you may recognize from your Task Manager, as it is responsible for the Real-Time protection feature that monitors web activity for any malicious threats. Due to a bug, Firefox was making more frequent calls to this service than other browsers like Chrome or Edge, which was causing significant CPU spikes. These spikes could have a negative impact on the performance of other applications or drain the battery life of a laptop.

This problem was initially reported on Mozilla's bug tracker system back in 2018, and was promptly attributed to the MsMpEng service. However, it was only after the diligent documentation efforts of Juglaret that Microsoft's developers took more immediate action. The updated version of Windows Defender has now been released for Windows 10 and Windows 11 users, and should be available to all (except those who intentionally block the update) within the next week or two.

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