Firefox has a reputation of being something of a resource hog, even among modern browsers. But it might not be entirely earned, because it looks like a CPU bug affecting Firefox users on Windows was actually the fault of Windows Defender. The latest update to the ubiquitous security tool addresses the issue, and should result in measurably lower CPU usage for the Windows version of Firefox.
According to Mozilla senior software engineer Yannis Juglaret (via Neowin), the culprit was MsMpEng.exe, which you might recognize from your Task Manager. It handles the Real-Time protection feature that monitors web activity for malicious threats. The bug was causing Firefox to call on the service much more frequently than comparable browsers like Chrome or Edge, resulting in notable CPU spikes. Said CPU spikes could reduce performance in other applications or affect a laptop’s battery life.
The issue was first reported on Mozilla’s bug tracker system way back in 2018 and quickly assigned to the MsMpEng service, but some more recent and diligent documentation on the part of Juglaret resulted in more swift action from Microsoft’s developers. The patched edition of Windows Defender is now rolling out to Windows 10 and Windows 11 users, and should hit all of them (unless they specifically block the update) within the next week or two.