A CPU bug affecting Firefox users on Windows was actually the fault of Windows Defender, which Vole has finally gotten around to fixing.
When the update is installed Firefox has measurably lower CPU usage.
According to Mozilla senior software engineer Yannis Juglaret, the culprit was MsMpEng.exe, which 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 resulting in notable CPU spikes. The CPU spikes could reduce performance in other applications or affect a laptop's battery life.
It has been around for a while the problem was first spotted on Mozilla's bug tracker system in 2018 and Vole told the MsMpEng service team to get onto it. More recently though Mozilla has provided Vole with better documentation on the flaw and Microsoft pulled its fingers out.